Generative Fill error (All target frames are blank)

"After Effects warning: All target frames are blank. The Content-Aware Fill layer generation for Comp (...) has been aborted."

This error do I get when trying to Generate Fill Layer.

My Comp has a hole to fill, so that is not the problem.

I trashed Preferences as suggested in another thread.

Any suggestions?

Meanwhile, what does this even mean: "All target frames are blank."?