Hey there Getter1,
Thanks for reaching out to Apple Support Communities. We understand you’re running into an unexpected error when using Keynote on your Mac. We’ll be happy to help.
It sounds like Keynote is trying to access a previous version of the document you are working on. “Every time you save a presentation, Keynote saves it as a new version. As a result, you can browse earlier versions, save a copy of an earlier version, or restore a presentation to an earlier version.” You can read more about this here:
Restore an earlier presentation version in Keynote on Mac
Boot your Mac into safe mode and see if the issue continues there:
Start up your Mac in safe mode
You may also want to ensure your Mac is up to date as well:
How to update the software on your Mac
We hope this helps.
Take care.