Settings form requires different fields

This looks like you haven't configured your Slack channel yet. Please go to /admin/config/system/push_framework/slack and provide the credentials there, so that the Slack library can use them to send messages on your behalf.

You are right. Obviously got distracted by creating the ECA model and forgot the backend.

Entered the token xoxb-VALUE but still not working. I even reinstalled the app/bot to my workspace.

Error details:

Failed execution of Nachricht an Slack (#allgemeines) (Activity_087i5gl) from ECA Neuer/aktualisierter Inhalt (process_jtcehec) for event Drupal\eca_content\Event\ContentEntityUpdate: No PSR-18 clients found. Make sure to install a package providing "psr/http-client-implementation". Example: "php-http/guzzle7-adapter"..
#0 SITEROOT/vendor/jolicode/slack-php-api/src/ClientFactory.php(31): Http\Discovery\Psr18ClientDiscovery::find() #1 SITEROOT/web/modules/contrib/pf_slack/src/Plugin/PushFrameworkChannel/Slack.php(44): JoliCode\Slack\ClientFactory::create('xoxb-6897326535...')
#2 SITEROOT/web/modules/contrib/push_framework/src/Plugin/Action/Notify.php(101): Drupal\pf_slack\Plugin\PushFrameworkChannel\Slack->send(Object(Drupal\user\Entity\User), Object(Drupal\node\Entity\Node), Array, 0)
#3 SITEROOT/web/modules/contrib/eca/src/Entity/Objects/EcaAction.php(102): Drupal\push_framework\Plugin\Action\Notify->execute(NULL)
#4 SITEROOT/web/modules/contrib/eca/src/Processor.php(188): Drupal\eca\Entity\Objects\EcaAction->execute(Object(Drupal\eca\Entity\Objects\EcaEvent), Object(Drupal\eca_content\Event\ContentEntityUpdate), Array)
#5 SITEROOT/web/modules/contrib/eca/src/Processor.php(146): Drupal\eca\Processor->executeSuccessors(Object(Drupal\eca\Entity\Eca), Object(Drupal\eca\Entity\Objects\EcaEvent), Object(Drupal\eca_content\Event\ContentEntityUpdate), Array)
#6 SITEROOT/web/modules/contrib/eca/src/EventSubscriber/EcaBase.php(76): Drupal\eca\Processor->execute(Object(Drupal\eca_content\Event\ContentEntityUpdate), 'eca.content_ent...')
#7 [internal function]: Drupal\eca\EventSubscriber\EcaBase->onEvent(Object(Drupal\eca_content\Event\ContentEntityUpdate), 'eca.content_ent...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#8 SITEROOT/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(142): call_user_func(Array, Object(Drupal\eca_content\Event\ContentEntityUpdate), 'eca.content_ent...', Object(Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher))
#9 SITEROOT/web/modules/contrib/eca/src/Event/TriggerEvent.php(73): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object(Drupal\eca_content\Event\ContentEntityUpdate), 'eca.content_ent...')
#10 SITEROOT/web/modules/contrib/eca/modules/content/src/HookHandler.php(297): Drupal\eca\Event\TriggerEvent->dispatchFromPlugin('content_entity:...', Object(Drupal\node\Entity\Node), Object(Drupal\eca\Service\ContentEntityTypes))
#11 SITEROOT/web/modules/contrib/eca/modules/content/eca_content.module(97): Drupal\eca_content\HookHandler->update(Object(Drupal\node\Entity\Node))
#12 [internal function]: eca_content_entity_update(Object(Drupal\node\Entity\Node))
#13 SITEROOT/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(426): call_user_func_array(Object(Closure), Array)
#14 SITEROOT/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(405): Drupal\Core\Extension\ModuleHandler->Drupal\Core\Extension\{closure}(Object(Closure), 'eca_content')
#15 SITEROOT/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(433): Drupal\Core\Extension\ModuleHandler->invokeAllWith('entity_update', Object(Closure))
#16 SITEROOT/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(251): Drupal\Core\Extension\ModuleHandler->invokeAll('entity_update', Array)
#17 SITEROOT/web/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php(900): Drupal\Core\Entity\EntityStorageBase->invokeHook('update', Object(Drupal\node\Entity\Node))
#18 SITEROOT/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(598): Drupal\Core\Entity\ContentEntityStorageBase->invokeHook('update', Object(Drupal\node\Entity\Node))
#19 SITEROOT/web/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php(781): Drupal\Core\Entity\EntityStorageBase->doPostSave(Object(Drupal\node\Entity\Node), true)
#20 SITEROOT/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php(523): Drupal\Core\Entity\ContentEntityStorageBase->doPostSave(Object(Drupal\node\Entity\Node), true)
#21 SITEROOT/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php(804): Drupal\Core\Entity\EntityStorageBase->save(Object(Drupal\node\Entity\Node))
#22 SITEROOT/web/core/lib/Drupal/Core/Entity/EntityBase.php(339): Drupal\Core\Entity\Sql\SqlContentEntityStorage->save(Object(Drupal\node\Entity\Node))
#23 SITEROOT/web/core/modules/node/src/NodeForm.php(270): Drupal\Core\Entity\EntityBase->save()
#24 [internal function]: Drupal\node\NodeForm->save(Array, Object(Drupal\Core\Form\FormState))
#25 SITEROOT/web/core/lib/Drupal/Core/Form/FormSubmitter.php(114): call_user_func_array(Array, Array)
#26 SITEROOT/web/core/lib/Drupal/Core/Form/FormSubmitter.php(52): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object(Drupal\Core\Form\FormState))
#27 SITEROOT/web/core/lib/Drupal/Core/Form/FormBuilder.php(597): Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object(Drupal\Core\Form\FormState))
#28 SITEROOT/web/modules/contrib/autosave_form/src/Form/AutosaveFormBuilder.php(144): Drupal\Core\Form\FormBuilder->processForm('node_article_ed...', Array, Object(Drupal\Core\Form\FormState))
#29 SITEROOT/web/core/lib/Drupal/Core/Form/FormBuilder.php(325): Drupal\autosave_form\Form\AutosaveFormBuilder->processForm('node_article_ed...', Array, Object(Drupal\Core\Form\FormState))
#30 SITEROOT/web/modules/contrib/autosave_form/src/Form/AutosaveFormBuilder.php(97): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\node\NodeForm), Object(Drupal\Core\Form\FormState))
#31 SITEROOT/web/core/lib/Drupal/Core/Controller/FormController.php(73): Drupal\autosave_form\Form\AutosaveFormBuilder->buildForm(Object(Drupal\node\NodeForm), Object(Drupal\Core\Form\FormState))
#32 [internal function]: Drupal\Core\Controller\FormController->getContentResult(Object(Symfony\Component\HttpFoundation\Request), Object(Drupal\Core\Routing\RouteMatch))
#33 SITEROOT/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array)
#34 SITEROOT/web/core/lib/Drupal/Core/Render/Renderer.php(580): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#35 SITEROOT/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure))
#36 SITEROOT/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
#37 SITEROOT/vendor/symfony/http-kernel/HttpKernel.php(169): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#38 SITEROOT/vendor/symfony/http-kernel/HttpKernel.php(81): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1)
#39 SITEROOT/web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#40 SITEROOT/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#41 SITEROOT/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#42 SITEROOT/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#43 SITEROOT/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#44 SITEROOT/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#45 SITEROOT/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#46 SITEROOT/web/core/lib/Drupal/Core/DrupalKernel.php(718): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#47 SITEROOT/web/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request))
#48 {main}

Maybe making the Token field mandatory would help avoiding such silly support requests.

Side notice: Slack module uses incoming webhook, which works just fine. So maybe I have not completed the configuration on the Slack end correctly?

Well, the error message

No PSR-18 clients found. Make sure to install a package providing "psr/http-client-implementation". Example: "php-http/guzzle7-adapter"

comes form \Http\Discovery\Psr18ClientDiscovery::find which is a third party dependency. Why they are missing a client, I can't tell, that's probably something to report to the maintainer of the jolicode/slack-php-api package which is used to talk to Slack.

I don't have any access to a Slack instance where I could get webhook credentials from, so I can't test that myself. But I'm happy to review and merge MRs, if you can fix that at your end.

Just checked on their readme at https://packagist.org/packages/jolicode/slack-php-api and they advise to install a PSR-18 client or PSR-7 message factory if none does exist in your project yet.

As I'm running this in Drupal 7, I suspect that the symfony/http-client fulfils that requirement. I'm not sure if the same applies for Drupal 9 and I don't know which Drupal version you're using.

Maybe these hints are getting you further into the debugging of this.

For the jolicode part: Would that only be a dependency to add?

For webhook: Seems like webhooks may be deprecated in a while, that's why there is a rewrite for version 2.0 of the Slack module (https://git.drupalcode.org/project/slack) which as far as I can see comes without any additional dependency. They even claim "You can also use our module API in your modules" in their Readme. May be worth a look.

I am using Drupal 9.5

Tried requiring symfony/http-client (ok).
With a renewed bot configuration, it worked.

You might consider adding the dependency.