I already paid for Microsoft office. Why can't I open after software upgrade

If your paid for version of MS-Office is an older (32-bit) one, it won't run under the newer operating systems, such as 10.15 or 11, which only run 64-bit programs. Also, Microsoft has dropped support for those older versions. There are other older programs that people may have paid for but can't be run under newer operating systems. This phenomenon also occurs frequently with Windows computers, it is not just Macs.

Both Apple and Microsoft have posted notices for several years documenting these things. You are not the only person who was surprised by this, however. Not much we can do about it as Apple has moved to the 64-bit environment and Microsoft did not offer a free upgrade from its 32 bit older MS-Office software to a 64 bit version.

You can either pay to upgrade to the newest or newer version of MS-Office, or try using Pages/Numbers/Keynote, which Apple provides free on its computers. Pages/Numbers/Keynote can open and save files in MS-Office format. There are also other free options such as Open Office you can try.