Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
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.
I explained in greater details to this OP here as a lot of follow up questions were asked. In case you want some further reading: