andreea232,
If this happening only with this one file, then the situation would be isolated to the file. If it's happening with all Pages documents saved to your Files app/iCloud Drive then a bit further isolating would need to be done. For example, if you create a new document in Pages on your Mac, are you able to use that file or does a similar situation occur? If a new file is created on a different device and you try to open it on your Mac does the same error occur?
At this point though, we are going to recommend you reach out to Apple Support and work with them directly on the matter, as we want to be sure all bases get covered to get you back in your document.
You can reach out with this link: Apple Get Support and start with Music, Apps & Services > Mac apps.
Thank you.