If you're unable to take class attendance, there could be a few reasons.
If you expect to see choices like "Tardy" or "Absent" when you click into the menu, but see nothing, then the issue is that attendance types have not been configured to display on the Class Attendance page. This can be corrected by editing the "For Class" setting on attendance types.
Another issue may be that there simply is no menu at all. If this is the case, the issue is that the section has been marked as ineligible for attendance. If you use PowerSchool as your SIS, the root cause is almost certainly found there. Please update the "Exclude From Attendance" setting for the section in PS.
If you see that an attendance type is unchangeable and has a lock icon next to it, the issue is that someone with more permissions than you has set this code. The solution is to contact the person who set the attendance (hover over the attendance type to learn who this was) or to request the "Lock Class Attendance" permission yourself. More here.