Error: Class "MailjetTools\MailjetApi" not found in Drupal\mailjet\Form\MailjetApiForm->submitForm()

here is a patch

This patch works perfectly,

Thank you,

This is not the correct way to fix this. Because this is a library downloaded by the composer mailjet/mailjet-apiv3-php

We need to add the following to the composer.json file

"MailjetTools\\": "docroot/modules/contrib/mailjet/lib/mailjet-api-php/"

in the section

 "autoload-dev": {
        "psr-4": {
            "Drupal\\Tests\\PHPUnit\\": "tests/phpunit/src/",
            "MailjetTools\\": "docroot/modules/contrib/mailjet/lib/mailjet-api-php/"
        }
    },

Then refresh the autoload path by executing the command

composer dump-autoload

This should fix the error.