The document could not be autosaved. The file has been changed by another application.

I found a solution:

In System Preference -> Security & Privacy -> Full Disk Access

Add Keynote, or others App affected

Close app and reopen. Try save and message disappeared!

This doesn't help. I keep getting the same alert with Pages.

I've discovered a FIX for my situation!!

I store most of my files on Google Drive and as such I receive this dreaded error on Keynote, Pages, etc. This was happening as Google Drive (and other similar online file storage companies like Dropbox) was accessing the file to back it up and store it offline but I was still working on the file so I would receive the error often while working which was a huge disruption.

My fix was to select the files I was working on and right-click, go to the Google Drive menu and select these files to be available offline (so they wouldn't be trying to make them offline which is my default). This got rid of the errors for me. Hope this helps someone else.

Pages autosave is designed to work with local filesystems, iCloud Drive, or USB mounted drive in a supported filesystem format. Apple did not support autosave to work on Google Drive, so move that Pages document back to your Mac or iCloud filesystem to avoid this error.

Google Drive will ignore the fact that Microsoft documents are zip containers, and not attempt to unzip them, but Apple's Pages, Numbers, and Keynote documents are perceived as just zip files on Google Drive, and not as real documents. If you were to double-click Apple's documents, Google Drive would attempt to unzip (and destroy) them.

I had a similar experience. The problem eventually turned out to be incompatibility with the server, which was a 2010 Mac mini server and no longer upgradable. I found out during the process of getting a new Mac mini that file sharing was removed from server functions and I no longer needed the server so I returned the Mac mini and now store my files in iCloud. The upshot, was the problem disappeared.

That's interesting but I have this same problem with a Numbers file that I just created on my new MacBook Pro 2021. Any suggestions?

I already had hope but no: It does not work in my case.

This really seems to be an ongoing problem for many mac users who use filesharing apps such as google drive or dropbox.