On my Mac mini Late 2012, I had upgraded the OS to Catalina but found I can't use the most important of my apps. I wanted to downgrade to Sierra using the advice here, when I go to the App folder, click Install macOS Sierra it says This copy is too old on this version of macOS
You can not simply go back by installing Sierra or High Sierra.
First off, it requires a backup of what you were running before you upgraded to Catalina. That way you can restore your data back the way it was in the earlier macOS.
Next it requires using internet recovery to completely erase, reformat the internal drive, then you can reinstall the earlier macOS version and restore from the earlier backup.
A one simple solution to your problem is to get an external drive and install Sierra on it and then install those apps that only work with Sierra.
The other solution if you have enough room on your internal drive, add a new partition on your internal drive and and install Sierra and apps on it.
Yet another solution is using a virtual machine and install Sierra on it. Check with Parallels Desktop Mac or VMware Fusion for an explanation of how it would work in detail. Basically, it is like running a Mac as an application inside you Mac.
Third, upgrade you "most important" app to a version that is compatible with Catalina or search for a suitable alternative. This is probably the wisest approach because as time goes on, the situation will just worsen and that 2012 MacMini will not last forever.
Trying to go from Catalina to Install macOS Sierra it says "This copy is too old..."
