Error: Class "\Mailchimp\Mailchimp" not found in Drupal\mailchimp\ClientFactory->createInstance() (line 161 of modules/contrib/mailchimp/src/ClientFactory.php).

Any idea how to solve that?

Do you have a folder vendor/thinkshout/mailchimp-api-php? This should be downloaded when you run composer require drupal/mailchimp:^2.2 or composer install any point after that.

Have you verified that the latest version of the mailchimp module has been downloaded to your installation?

When and where are you seeing the notice about package mailchimp/mailchimp? That should not be needed at all.

Yes I have this modul installed:
composer require drupal/mailchimp:^2.2
./composer.json has been updated
Running composer update drupal/mailchimp
Loading composer repositories with package information
Updating dependencies
Nothing to modify in lock file
Writing lock file
Installing dependencies from lock file (including require-dev)
Nothing to install, update or remove
Generating autoload files
93 packages you are using are looking for funding.
Use the `composer fund` command to find out more!
phpstan/extension-installer: Extensions installed
Scaffolding files for drupal/core:

And in the vendor directory:
ls -l vendor/thinkshout/mailchimp-api-php/
total 24
-rw-r--r-- 1 uid81104 gid81104 612 déc 8 2023 composer.json
-rw-r--r-- 1 uid81104 gid81104 3150 déc 8 2023 CONTRIBUTING.md
-rw-r--r-- 1 uid81104 gid81104 268 déc 8 2023 phpunit.xml
-rw-r--r-- 1 uid81104 gid81104 3966 déc 8 2023 README.md
drwxr-sr-x 3 uid81104 gid81104 4096 déc 8 2023 src
drwxr-sr-x 3 uid81104 gid81104 4096 déc 8 2023 tests

The module is there, but when I go to the "admin/config/services/mailchimp", I have this error:
The website encountered an unexpected error. Try again later.

Error: Class "\Mailchimp\Mailchimp" not found in Drupal\mailchimp\ClientFactory->createInstance() (line 161 of modules/contrib/mailchimp/src/ClientFactory.php).

super strange

this is my composer.json:

{
    "name": "drupal/recommended-project",
    "description": "Project template for Drupal 9 projects with a relocated document root",
    "type": "project",
    "license": "GPL-2.0-or-later",
    "homepage": "https://www.drupal.org/project/drupal",
    "support": {
        "docs": "https://www.drupal.org/docs/user_guide/en/index.html",
        "chat": "https://www.drupal.org/node/314178"
    },
    "repositories": [
        {
            "type": "composer",
            "url": "https://packages.drupal.org/8"
        },
      {
        "type": "package",
        "package": {
          "name": "northernco/ckeditor5-anchor-drupal",
          "version": "0.2.0.",
          "type": "drupal-library",
          "dist": {
            "url": "https://registry.npmjs.org/@northernco/ckeditor5-anchor-drupal/-/ckeditor5-anchor-drupal-0.2.0.tgz",
            "type": "tar"
          }
        }
      }
    ],
  
    "require": {
        "composer/installers": "^1.9",
        "drupal/anchor_link": "^3.0@alpha",
        "drupal/antibot": "^2.0",
        "drupal/backup_migrate": "^5.0",
        "drupal/better_social_sharing_buttons": "^4.0",
        "drupal/ckeditor": "^1.0",
        "drupal/ckeditor5_findandreplace": "^1.1",
        "drupal/ckeditor5_plugin_pack": "^1.2",
        "drupal/ckeditor_find": "^2.0@beta",
        "drupal/ckeditor_font": "^2.0@beta",
        "drupal/content_entity_clone": "^1.0",
        "drupal/core-composer-scaffold": "^10",
        "drupal/core-project-message": "^10",
        "drupal/core-recommended": "^10",
        "drupal/datalayer": "^2.0@RC",
        "drupal/editor_advanced_link": "^2.2",
        "drupal/elasticsearch_connector": "^7.0@alpha",
        "drupal/elasticsearch_search_api": "^2.1",
        "drupal/entity": "^1.4",
        "drupal/entity_clone": "^2.0@beta",
        "drupal/entityqueue": "^1.6",
        "drupal/facets": "^3.0@beta",
        "drupal/facets_date_range": "^1.0@beta",
        "drupal/fakeobjects": "^1.2",
        "drupal/flexible_permissions": "^1.0",
        "drupal/image_widget_crop": "^2.4",
        "drupal/imce": "^3.0",
        "drupal/linkit": "^6.1",
        "drupal/ludwig": "^2.0",
        "drupal/mailchimp": "^2.2",
        "drupal/metatag": "^2.0",
        "drupal/migrate_plus": "^6.0",
        "drupal/migrate_tools": "^6.0",
        "drupal/migrate_upgrade": "^4",
        "drupal/paragraphs": "^1.15",
        "drupal/pathauto": "^1.11",
        "drupal/redirect": "^1.9",
        "drupal/scheduler": "^2.0",
        "drupal/search_api": "^1.29",
        "drupal/search_api_page": "^1.0",
        "drupal/search_api_sorts": "^1.1",
        "drupal/simple_sitemap": "^4.1",
        "drupal/twigsuggest": "^1.0@RC",
        "drupal/upgrade_status": "^4.0",
        "drupal/views_data_export": "^1.3",
        "drupal/views_migration": "^1.2",
        "drupal/webform": "^6.2",
        "northernco/ckeditor5-anchor-drupal": "^0.2.0",
        "thinkshout/mailchimp-api-php": "v3.0.1"
    },
    "conflict": {
        "drupal/drupal": "*"
    },
    "minimum-stability": "dev",
    "prefer-stable": true,
    "config": {
        "allow-plugins": {
            "composer/installers": true,
            "drupal/core-composer-scaffold": true,
            "drupal/core-project-message": true,
            "dealerdirect/phpcodesniffer-composer-installer": true,
            "phpstan/extension-installer": true,
            "php-http/discovery": true
        },
        "sort-packages": true
    },
    "extra": {
        "drupal-scaffold": {
            "locations": {
                "web-root": "web/"
            }
        },
        "installer-paths": {
            "web/core": [
                "type:drupal-core"
            ],
            "web/libraries/{$name}": [
                "type:drupal-library"
            ],
            "web/modules/contrib/{$name}": [
                "type:drupal-module"
            ],
            "web/profiles/contrib/{$name}": [
                "type:drupal-profile"
            ],
            "web/themes/contrib/{$name}": [
                "type:drupal-theme"
            ],
            "drush/Commands/contrib/{$name}": [
                "type:drupal-drush"
            ],
            "web/modules/custom/{$name}": [
                "type:drupal-custom-module"
            ],
            "web/profiles/custom/{$name}": [
                "type:drupal-custom-profile"
            ],
            "web/themes/custom/{$name}": [
                "type:drupal-custom-theme"
            ]
        },
        "drupal-core-project-message": {
            "include-keys": [
                "homepage",
                "support"
            ],
            "post-create-project-cmd-message": [
                "<bg=blue;fg=white>                                                         </>",
                "<bg=blue;fg=white>  Congratulations, you’ve installed the Drupal codebase  </>",
                "<bg=blue;fg=white>  from the drupal/recommended-project template!          </>",
                "<bg=blue;fg=white>                                                         </>",
                "",
                "<bg=yellow;fg=black>Next steps</>:",
                "  * Install the site: https://www.drupal.org/docs/installing-drupal",
                "  * Read the user guide: https://www.drupal.org/docs/user_guide/en/index.html",
                "  * Get support: https://www.drupal.org/support",
                "  * Get involved with the Drupal community:",
                "      https://www.drupal.org/getting-involved",
                "  * Remove the plugin that prints this message:",
                "      composer remove drupal/core-project-message"
            ]
        }
    },
    "require-dev": {
        "drupal/core-dev": "^10"
    }
}

I have removed them and reinstall them, and still same error:
composer require drupal/mailchimp
./composer.json has been updated
Running composer update drupal/mailchimp
Loading composer repositories with package information
Updating dependencies
Lock file operations: 2 installs, 0 updates, 0 removals
- Locking drupal/mailchimp (2.2.4)
- Locking thinkshout/mailchimp-api-php (v3.0.1)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 2 installs, 0 updates, 0 removals
- Downloading drupal/mailchimp (2.2.4)
- Installing thinkshout/mailchimp-api-php (v3.0.1): Extracting archive
- Installing drupal/mailchimp (2.2.4): Extracting archive
Generating autoload files
93 packages you are using are looking for funding.
Use the `composer fund` command to find out more!
phpstan/extension-installer: Extensions installed
Scaffolding files for drupal/core:

Ok I had some progress here, I finnally make this working, the compser didn't wanted to the end because he didn''t had the permission to write on the default folder, so now the module is installed, but I have another error, I'm unable to login:

I did the Oauth authentificaiton, and in mailchimp in integration I got the new ligne ast he app is integretated correctly, but I still have the message that the oauth is not setup:
Unable to connect to Mailchimp API. Visit OAuth Settings page to authenticate or uncheck "Use OAuth Authentication" and add an api_key below (deprecated).

Ok I used the api key (deprecated) and it's working well