Why can't I parent objects in my .blend? Here's a weird one - I can't seem to create a parent child relationship in my current Blender project. 

For example, I want to make 'Cube' a child of the 'ModuleParent' empty. If you take a look at this linked .blend, can you create that relationship? 

If so, how? (Have tried ctrl-p, have tried dragging and dropping in Outliner (with shift and ctrl), nowt seems to work)

Thanks in advance for your help.

--Rev
Set outliner display mode to Scene

I wonder if this is the outliner display mode being viewlayer rather than scene as default. Was a source of confusion for me the other day as  parent relationship works, but in viewlayer display doesn't show as hierarchy like we are used to in 2.79 and prior.  

In your file cube is a child of another cube that is a child of the empty. Can drag around in outliner after setting display mode to scene and parenting works as in previous versions with drag and drop.