Can't open Excel files after upgrading to Big Sir on my iMac from Catalina

That might not be the best advice until you understand whether they have a Microsoft 365 subscription or the single-purchase Office 2019 for Mac. The Mac App Store applications are strictly subscription versions and would overwrite the single-purchase license applications, causing license mayhem.

I would check Office for Mac and verify you are running the most up-to-date version. If you are please do the following:

Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

Have you tried dragging the Excel app (not your spreadsheets) to the Trash, then getting Excel from the Mac App Store?

Cheers,

Jack

What happens when you try?

What version of Excel do you have?

Which version of Office for Mac is installed on the computer? This is critical so please answer.