Can't make clipping mask?
The top selected object must be a path, a compound shape, a text object, or a group of those.
As usual, Adobe's so called "Help" is anything but.

"Only vector objects can be clipping masks; however, any artwork can be masked."
Well, what is a Rectangle? Not a vector?
Why on Earth would Illustrator not let a Rectangle be used for a clipping mask?

Why would this be active if it is not a possible operation?

Who's idea was it to make simple operations baffling, impossible and openly hostile to users trying to get some work done?
Why would this not be a simple and quick function?
Can anyone at Adobe explain this philosophy?
