A family of Microsoft word processing software products for creating web, email, and print documents.
If your OS is above version 12 but below 14, your office products can be functional, but Microsoft no longer maintains or updates that version. In this scenario, you can install the older downgraded version.
However, if it is below 12, it will now enter reduced functionality mode. In another word, the OS is too old to receive the licensing update the Office now requires. Sorry. Buying new Office will not help. It is the MacOS being the problem, not office.