Can't create a mask from a path!

As per your screenshot you are trying to make a layer clipping mask. That works a bit different. You'd have to highlight the corresponding layer and then make the mask in the Layers palette. Note that the top object (path) in the stacking order will be the clipping path.

To make a clipping mask you need to select 2 objects, the object to be masked and above a path to mask the object below.

Object menu > Clipping Mask > Make