Why can't I find some courses when I try to create an assignment in my group?
The course you are referring to is likely a course that has not been formally released yet. We call these "soft-launched" courses.
Soft-launched courses are still being monitored for quality purposes, therefore they cannot yet be used for assignments; alternatively, admins can instruct their members to independently complete soft-launched courses, as they're still available via DataCamp's search bar.
Note: Open courses are not official DataCamp courses, so they would not be listed in the Assignments dropdown menu.