Some troubleshooting steps you might consider:
Try creating a new document, store it in Dropbox, and see if you can open that.
Then trying moving it out of the Dropbox folder onto your local drive and see if you can open it there.
Try creating a new document, then save it on your local drive and see if you can open it then.
If documents not stored in Dropbox open as expected but not those stored in Dropbox then you might consider storing them in iCloud Drive instead. Numbers is designed to work well with iCloud.
Check your macOS version. The latest version of Big Sur is 11.5.1 so you should update to that if you are running Big Sur. Go to System Preferences > Software Update to do that. If you are on an older macOS then update to the latest available version that Software Update permits.
Sometimes older Macs have two versions of Numbers. Make sure you are using 11.1 and not the older one by launching the version 11.1 directly and then going to File > Open and navigate to a document to open it (as opposed to double-clicking the document in Finder).
And don't forget to try checking your internet connection and restarting your Mac. That sometimes can clear up mysterious problems.
SG
