I can't edit my project in progress.

OK - since you are using the mimeo plug in, I'd suggest talking to them. They will be best placed to help you get it working again.

The only thing I've found relating to Mimeo on Big Sur is this:

https://intercom.help/mimeo-photos/en/articles/5402976-project-images-are-missing-after-icloud-sync

Does that sound like your issue?

Hi

Can you tell us which version of MacOS you are using, and what plugin you are using for the photo book. Apple no longer provides photobooks, but the apple plugin is still available on older versions of MacOS leading to the problem that people create books that can not be printed.

You will need to use a third party plugin that is still supported. One I've used is Motif - it works fine, but I think for me a better approach is to use a third party standalone app, rather than a Photos plug in. I will be going back to Blurb in the future.