If you have an older version of Office, like Office 2011, your Office won’t work with BigSur (nor did it work with Catalina). Older versions are 32-bit apps and Catalina and BigSur need 64-bit apps.
You can either subscribe to Office 365 or buy Office 2019
There are free alternatives:
- if you have an outlook.com account, you can use the webversion of Office (go to their website)
- try the free OfficeLibre for Mac that works really fine together with Office files https://www.libreoffice.org/discover/libreoffice/
- Apple’s own Pages, Numbers and Keynote (already on your Mac)