Semester enrollment (in courses) can be blocked by the action of various administrative offices. This is done to ensure that your records are properly maintained.
If you have holds, they will be listed on the Student - Semester Enrollment screen (under "Academics) on TigerNET. Holds are listed in the Schedule Entry block at the bottom of the page as illustrated below.
Note: You must be logged into TigerNET to see your holds, if any.