Class 'PHPMailer\PHPMailer\PHPMailer' not found in Drupal\mailjet\Plugin\Mail\MailjetMail->mail()

The older versions was requiring manual installation of PHPMailer. The 3.0.x-dev already includes the requirement in composer.json so it would be installed automatically.

SocialNicheGuru, can you please confirm how did you install the module?

Yes I added it via composer, "drupal/mailjet":"3.0.x-dev@dev",

message:
- Installing drupal/mailjet (dev-3.0.x 68b900f): Cloning 68b900f5d4 from cache

The website encountered an unexpected error. Please try again later.
Error: Class 'PHPMailer\PHPMailer\PHPMailer' not found in Drupal\mailjet\Plugin\Mail\MailjetMail->mail() (line 169 of modules/contrib/mailjet/src/Plugin/Mail/MailjetMail.php).
Drupal\mailjet\Plugin\Mail\MailjetMail->mail() (Line: 50)
Drupal\mailsystem\Adapter->mail() (Line: 307)
Drupal\Core\Mail\MailManager->doMail() (Line: 180)
Drupal\Core\Mail\MailManager->Drupal\Core\Mail\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 181)
Drupal\Core\Mail\MailManager->mail() (Line: 70)
Drupal\mailsystem\MailsystemManager->mail() (Line: 1070)
_user_mail_notify() (Line: 128)
Drupal\user\RegisterForm->save()
call_user_func_array() (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers() (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm() (Line: 592)
Drupal\Core\Form\FormBuilder->processForm() (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult() (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 54)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle() (Line: 58)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 43)
Drupal\webform_product\RedirectMiddleware->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 23)
Stack\StackedHttpKernel->handle() (Line: 719)
Drupal\Core\DrupalKernel->handle() (Line: 19)

Hi SocialNicheGuru, can you please check the following?

1. What is the phpmailer/phpmailer version you have installed?
lando composer show phpmailer/phpmailer
2. Make sure your composer autoloader is up to date; the easiest way to do that is to remove the vendor directory and run composer install.
3. In some rare cases, PHP interpreter opcache may be out of date. Try restarting your PHP process.

composer show phpmailer/phpmailer
name : phpmailer/phpmailer
descrip. :
keywords :
versions : * 6.0.7
type : drupal-library
homepage :
source : []
dist : [zip] https://github.com/phpmailer/phpmailer/archive/refs/tags/v6.0.7.zip
path : /mypath/html/libraries/phpmailer
names : phpmailer/phpmailer

html/libraries/phpmailer$ ls
COMMITMENT get_oauth_token.php LICENSE SECURITY.md VERSION
composer.json language README.md src

My php version is 7.4+

Is this a specific php8.0 syntax that is throwing an error?

It seems like the library is installed at the wrong path; now sure how that's possible but it's in libraries/phpmailer while it should be at vendor like any normal PHP library.