Why can't I drag my Blueprint components?

I am new to UE4. I started by following the “Vehicles in UE4” tutorial. I followed the instructions accurately and everything was going well. Until the step when we need to add the Camera and attach it to the SpringArm. The Camera is created but I can’t drag and drop it on the SpringArm. The component is simply un-draggable.

I also noticed that the “Compile” button is disabled. When I hover over it, a message “Dirty: needs to be recompiled” is displayed. I don’t know if these 2 issues are related but I noticed in the video tutorial that the Compile button is enabled.

What might be the problem here?

I restarted the editor and everything went fine. Strange!

Encountered the same problem today, it’s because the game or simulation is running in the viewport. Stop it and it should work again.