Hello. I have Photoshop 2021. I'm wondering why after adding a layer mask and looking in the layers panel, I see an added thumbnail with brackets around it to indicate the mask. Why don't I get a thumbnail showing what the mask looks like? Thanks
The "brackets" around your mask shows that it is targeted. This will happen if you click once on a layer mask. If you were to start painting on the canvas you would actually be drawing on that targeted mask.
Your lack of a thumbnail preview is an easy fix. In the upper right corner of the Layers panel is a menu that contains the entry Panel Options. Just set your Thumbnail Size to something larger.