This may happen when you are trying to change task status of child task or dependent task. There must be parent task which of that child task which needs to be completed first before starting of child task.
You can check the above easily in your Gantt chart. For this, you need to navigate to the Gantt chart section.
To understand the above situation you may refer the below illustration:-
If you observe in above image you can find that Task 7 that is ‘Prepared a monthly report’ is parent task and Task 9 that is ‘Check Report’ is a child task.
Unless the Task 7 is not closed you will not be able to change the status or start the task 9.
Hope this helps!