The fix for this issue is now available in the latest update of InDesign – that is, InDesign 15.0.2.
Please update your InDesign application to get this fix.
Adobe InDesign team
An error occurred while saving the commentVince commented
It's Feb. 5 2021, and my InDesign 16 files saved fine yesterday, although there was no undo, now today there's no Save functions at all and no Undo. That's why I'm here!!! When I get those pop-ups to rate whatever Adobe app I'm using I suggest that they STOP UPDATING SO OFTEN!!! I just want to create without having these ridiculous continuous updates tripping me up. I updated most of my apps a few weeks ago, including indesign and and now there's the 16.1 available and I keep getting that stupid message and today I have 12 new updates available. THIS IS INSANE AND DOES NOT HELP CREATIVES! JUST STOP UPDATING FEATURES AND FIX THESE ISSUES. I'm 8 hours into laying out a photography book and Save disappeared quite a while ago and thanks to the gentlemen in this conversation, I Exported as an IDML, and it opened and then the Save functions worked. ADOBE STOP THIS UPDATE MADNESS!!! Once or twice a year would be fine.