Keyframe a mask

Hi, i’m trying to keyframe a mask in order to animate it, then I follow this tutorial, minute 7:08

he aimate the mask (pput the keyframe) by hitting the letter “i”, but in mine does not work, I I have already activate the opcion in the preferences.

what do you think I can do, please?

what do you think I can do, please?

Supply a demo blend file that shows your issue so we have something to look at