Can't use "fill" color. Seems that "fill" and "stroke" have gotten themselves in a loop.

I have been using Illustrator CC 25.1 with my own workspace and today when I went to create a diagram I can no longer "fill" an empty object.  The "fill" color changes the "stroke" color but I can't for the life of me get a "fill" working.  I have tried using different "workspaces" but to no avail.  I am on a MacBook Pro 16inch Retina and the latest Illustrator CC 2021

Is anybody else experiencing this problem?  If not, can you suggest what I may do to rectify the matter please?

Thank you

Sabina