Photoshop (20.2.4) Gradient alpha/opacity not working

All of a sudden my alpha gradients quit working.  I can select a preset or make a custom alpha gradient in the gradient menu, but it doesn't actuate.  Instead, it creates a solid fill.

I found this problem using Photoshop CC 2019 (20.), so I uninstalled it and because of an old computer and an my essential but old tablet's incapatibility with v22.1,  I have installed 21.2.4 and the same problem is occuring.  This makes me think it's my Mac (MacOS 10.13.6)... any ideas?