Please check the steps mentioned below to debug the issue on your end:

1) Check if all functions mentioned in the documentation is added to the external services.
2) Check if you are using the Right Moodle URL which should be copy-pasted from the Address bar. It should not be an extended version of the URL e.g: https://moodledomain.com/courses/ It should not be used in this way. You need to use https://moodledomain.com/ as the Moodle URL.
3) If you are using https://www.moodledomain.com/ as the Moodle URL then try changing it to https://moodledomain.com/

Once these changes are done, try synchronizing the courses again.