Cannot save Indesign Files

Have you tried to trash the preferences on Mac.

https://www.macrumors.com/how-to/automatically-empty-trash-mac/

If files won't save or things are not working as expected - this can happen. It's all part of maintenance on your side. 

Try resetting your preferences:
The easiest way to reset everything is to delete the InDesign preferences: as soon as InDesign starts to boot hold down Shift, Alt, Ctrl (and Apple/Cmd if working on a Macintosh). A dialog will appear asking to delete the InDesign Preferences, select 'Yes' on this dialog.
Further info and instructions here if needed:
https://helpx.adobe.com/in/indesign/kb/indesign-preferences-support-file-locations.html

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If a document is not behaving well then try 

File>Export 
Choose IDML
Open the IDML in InDesign
Save the file as a new InDesign file with a new name
See if the issue persists.

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Reinstalling rarely fixes everything - you can do a complete reinstall using the
Creative Cloud Cleaner Tool https://helpx.adobe.com/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

Hi @defaultcywucj5zceim,

Does this happen with just a single file or all files? If it's all files then maybe this is an issue with access to disk for InDesign program

https://iboysoft.com/wiki/full-disk-access-mac.html

If it happens with a single file, then probable exporting it to IDML and then trying might help.

Try trashing your preferences if the above steps don't work.

https://www.rockymountaintraining.com/adobe-indesign-rebuilding-preferences-cache/

-Manan