Upgrade to High Sierra Issue
I have an iMac, vintage 2012, that was running High Sierra. I needed to erase the HD because it started having issues (running super slow), and I reinstalled the OS through Recovery Mode. Unfortunately, it went back to its original OS, Mountain Lion. Now it won't let me upgrade from Mountain Lion to High Sierra. I've tried downloading Mountain Lion but the dmg file won't open and it won't let me access it through the App Store. Help!
iMac 21.5″, macOS 10.14