Problem in downgrading Mac OS

Hi All,

Really struggling to downgrade from Catalani macOS to high sierra which is the best last Macos system compatible with my I Mac 2011, Icore 7, processing machine with 1TB HD and 16 GB Ram.

On some online search did tried to upgrade to the Catalani Macos with some patch software, which did work without issue but left the Roblox game non-usable.

As a result of this tried to downgrade back to Mac OS high sierra but guess have done something extremely wrong.

I am now getting a question mark only on-screen and when trying to download Mac OS Lion only option available in restoring its apple ID doesn't let me in for some reasons, though the ID works on my MacBook Pro.

Quite frustrated don't want to do something more stupid am seeking to advise what to do further in restoring back to high Sierra and get rid of this current problem