upgrade

Yes, High Serra MacOS 10.13.6 is the tops for us 2011 iMac users...

Try installing this in Safe Mode, but not using Firefox as it’ll mess things up…

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

Safe Boot, (holding Shift key down at startup).

Only as far as High Sierra using only the Safari browser to reach the High Sierra Mac App Store download page in How to get old versions of macOS.