The error message is unambiguous. Your Mac's OS is not sufficient to run - or at least install - Office 365.
Microsoft's system requirements state that a Mac must be running the current or the two previous operating systems, which at the moment means 10.15 10.14 or 10.13.
Shortly this will be bumped up a notch when Big Sur Mac OS 11 is released. Apple hasn't indicated any specific date for this other than "Coming this Fall" - which the rest of the world calls Autumn, and here in the southern hemisphere is Spring.