I was able to create and save the Color Table without error. I use the command Image > Duplicate, changed the mode to Indexed and create the color table from the duplicate. See if that helps.
Other suggestions would be to not save to an external or network drive, turn off anti-virus.
And give Photoshop Full Disk Access: I would suggest a Mac restart when you change those settings.
Finally if you have Dropbox backup, it would be a good idea to disconnect it.
That's all I can think of, I hope others can chip in.
thanks - I don't have dropbox backup but it was a good idea indeed
thanks already tried that and it didn't fix it
still doesn't work unfortunately
Yes, sorry I see where you already tried full disk access. I guess you already reset preferences as well.
If not one shows up, try Support Chat: https://helpx.adobe.com/contact.html
There a bubble in the lower right that connects you to chat. Also try typing "agent" if you don't like answering screening questions.
thanks will try that