Hello

I am applying a Freeze Frame to the end of a clip with Ken Burns crop, but nothing happens, isn't it supposed to work?

You can do this 2 ways

1. Just park the playhead over the frame you want frozen, Choose Add Freeze Frame (Edit Menu or Option-f)
This inserts a separate clip (after the playhead) of the frozen frame that can be edited as you wish.

2. You can also do it as a variation of slow motion.
Select clip, park playhead on the frame, choose Hold from the slow motion dropdown (right end of the lighter gray toolbar just below the Inspector panel). This adds a red Hold bar above the clip that can be stretched out, etc. as needed.

One caveat - neither method works on the absolute last frame of your Project Timeline. These commands cannot extend a timeline for some reason, so you'll need to insert a Gap or Placeholder clip which can be deleted after adding the Freeze Frame.

I didn't know about the Hold option, thank you.
But I actually found the solution, I didn't know the Freeze Frame default length was tied to the Still Image length in Preferences. I had it set to one frame... :)