macOS upgrade from Catalina to Big Sur

The late 2015 is supporting Monterey. Please go to apple logo in menu bar>about this Mac to see if you really have a late 2015, and not a late 2013 (the late 2013 cannot update further then Catalina)

If you indeed have a late 2015 please try and update via this link

How to download and install macOS - Apple Support

You can go as far as Monterey OS 12.

And yes, Office will run just fine ... or should. My Late 2015 21.5" Retina iMac runs Monterey and Office 2016 beautifully.