Having the same problem.
New model, 4copies each of 3 materials, one is transparent, all relate to a live component window of which I have 3 copies in the model.

Mac OS Mojave 10.14.6
SU 2021 21.0.392

/Users/******/Library/Application Support/SketchUp 2020/SketchUp/Styles

Tried that, I'm still having no success.
I'll try restarting and recheck what I'm doing and digging a little further.
Thanks for your help in this case, it's the first time we have conversed, but I have appreciated your sound advice over the years as I have followed other Forum threads.
I'll let you know how I get on.

Yes, that is the route as shown when I try via right click- save as

Sorry for delayed response, family and work interuptions. Lockdown is strange.
Trying to save to Library/Application Support/SketchUp 2020/SketchUp/Styles.
Have tried drag and drop, the green+ indicator appears, but no copy forms in window and I cannot find a copy in the folder( or by attempting find on the computer)

Not sure what is meant by that, can you elaborate?

Just tried saving a new style (for the first time) by the right click method and getting the same 'Error saving the style file. The file could not be saved.' message
OS 10.14.6 SU 20.1.228

How about SU for the iPad Pro ?

Extract from UK supplier mail shot.
"
Is this general or UK only, and why so large an increase ?

Sketchup price rise.pdf

Thanks Dave for the answer, so simple.
More thanks to Edson for asking the question !