Most likely it is due to an attempt already having been made on the assignment. Once an attempt has been made, question content is locked. You have several options depending on your situation.

Need to ADD questions?  

Create a separate assignment for the new question(s).

Need to DELETE questions?  

Make the unneeded question(s) worth 0 points, thereby not affecting grades.

Delete previous attempts.  

If previous attempts are deleted, you will be able to add or delete questions on the assignment. 

WARNING!  Deleting an attempt is a PERMANENT action and content cannot be recovered.

If you are developing a new assignment where the attempts have just been trials made through a student account, then deleting the previous attempts will have no consequences in the gradebook. 

• We encourage you to use the Preview feature to attempt assignments; attempts will not lock in question content.

If you need to delete student attempts, think carefully or reach out to Instructor Support for guidance as to how to best handle your situation.