GraphicsGrid $\to$ Beep “The shared stylesheet “Stylesheet-review-topics.nb” could not be \ found. The default stylesheet will be used instead.”

12.0.0 for Microsoft Windows (64-bit) (April 6, 2019)

I have a notebook in which using GraphicsGrid causes a beep "The shared stylesheet "Stylesheet-review-topics.nb" could not be found. The default stylesheet will be used instead."

I copied the entire content of the notebook to a new one, set the style sheet to the one shown, and saved over the original notebook. Same thing happens. I created a new notebook and set the style sheet to the one shown, and it does not beep. My notebooks are fairly large, and I really don’t want to spend a lot of time untangling what amounts to a nuisance. I also doing like being annoyed.

This kind of think happens from time to time, and I’m never sure why. Any ideas?