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 38 that is ‘Website banner’ is parent task and Task 45 that is ‘Marketing template’ is a child task.
Unless the Task 38 is not closed you will not be able to change the status or start the task 45.
Hope this helps!