Here we have a video on how to install Office 365 on macOS Big Sur which is now classed as legacy and therefore the current version of Office 365 downloaded from the portal will not install on Big Sur Operating systems.
Here is some useful links:
MS Office compatible chart with macOS https://macuser.org.uk/2024/11/15/ms-…
Update history for Office for Mac (Download legacy versions of Office 365)
https://learn.microsoft.com/en-us/off…
Walk through of installing office 365 on legacy version on macOS https://macuser.org.uk/2024/11/08/off…