Saving Office for MAC Docs

I just had data migrated from my old iMac which was operating Catalina to the new iMac with the M1 chip and Big Sur. I had been using a purchsed copy of Office for Mac. and storing my files on the iMac but mostly in iCloud. When I opened a Excel document on my new iMac and edited it, I was unable to save it. Both the Save and Save As options are greyed out, as is the saved icon in the ribbon bar. In fact, I can't even close it without choosing the don't save option. Same thing with new docs. I can create but not save. I gave a yahoo email address when I bought the Office program, and it now appears that Yahoo/Microsoft somehow wants me to use OneDrive for cloud storage. I do not want to use OneDrive.

How can edit old documents and save new ones?

I contacted Apple Support, and they said it is a Microsoft problem, but I do not agree since these difficulties anly arose when I had the data migrated.