Hi killerlisabee,
Thank you for posting in Apple Support Communities. From our understanding, you are attempt to free up space in iCloud using your MacBook Air and are getting an error message that the original file cannot be found, do we have that right? Is this happening when dragging a specific file or several files? Where exactly are you trying to open up the file from? Are you signed into your iCloud account on your Mac in System preferences? Does this continue if you completely close out Finder, restart your Mac and try again? Also, be sure your Mac is running the latest version of macOS Big Sur.
You can also check out this article for the steps on how to copy files from iCloud Drive to your Mac: Archive or make copies of the information you store in iCloud.
Hope that helps!