Is MacOS Ver 12.0 available for earlier Mac's running on 11.7.10

What model year is your iMac? The model year of an iMac is available at Apple menu > About This Mac.

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. 

A late 2015 or mid 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. 

For all of the above, see instructions in: How to download and install macOS - Apple Support  

A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See:How to upgrade to macOS Sonoma - Apple Support

If your tax software is TurboTax, you can use TurboTax Online with much older Macs. I use it with my new Mac and it works very well. I prefer it to the installable version.

If you are using some other tax software, they may have a similar option.

Check Settings > Software Update to see if the version of Mac that you have has any available updates.

If not, unfortunately the tax software will not run on Big Sur, but you still may be able to run an online version of the tax software.