I had 12.5 up and running in 10.15.7 on Mac Mini late 2012. It was installed via series of upgrades. I cannot install it directly from AppStore.
But here is another problem, I cannot run SwiftUI application and SwiftUI preview. I downgraded XCode to 12.4 and a problem still the same.
KoCMoHaBT
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Nizhny Novgorod, Nizhny Novgorod Oblast, Russia