I suggest removing this field from your join table. You do not need it. Replace it with the same field from Courses.
Your relationship between Join and Courses should be:
Courses::__pkCoursesID = Join::_fkCoursesID
You don't have to name the fields like I have, but __pkCOursesID should be an auto-entered serial number that never changes and _fkCOursesID can be formatted with a value list of course IDs and course names so that you select the course by name, but the value list enters the ID number.
Thanks for the reply, its taken me sometime to get this sorted and I had to learn a bit more about filemaker. With your above post and another of your posts I managed to get it sorted. Thanks