Hi Richard!
You should access to the course as admin or editing teacher and review the configuration from the Participants page. You'll find more information at https://docs.moodle.org/en/Enrolment_methods#Managing_enrolment_methods
Hope this helps. However, if you don't find here the answer, please provide detailed information such as your role in this course, the enrolment method (manual?)...
Cheers,
Sara