I tried to downgrade my MacOS version on iMac

First, why would you want to downgrade? Normally (in almost ALL circumstances) this is a very poor idea.

Also, you don't state what model year iMac you own. If the computer did not originally ship with High Sierra or earlier you cannot install it. In short an earlier version of Mac OS cannot be installed a Mac than the version of Mac OS that it originally shipped with. This has been the case for over 20 years.

Finally, you did not detail the methodology you used when you attempted to downgrade so you are leaving us with no clues.

First, backup your data.

Then create a bootable macOS High Sierra USB.

After that, boot into macOS Recovery and erase the internal SSD.

Lastly, boot your Mac from USB and start installing macOS High Sierra.

The bootable USB installer will have the tools necessary -Disk Utility- to erase the internal drive. There will be no need to boot into Recovery.

Down-grading and going back is very difficult and not recommended.

If you are currently having trouble with macOS 10.15.7 Catalina...?

Then there is a great tool called EtreCheckPro that can help find both software and hardware issues.

To get and run the free version of EtreCheckPro, go to > https://etrecheck.com/en/index.html

To post your EtreCheckPro Report, see > How to use the Add Text Feature When Post… - Apple Community

heajung273 wrote:

tried to down grade from macOS catalina 10.15.7 to macOS High Sierra

Is this about running 32-bit applications? Or about trying to use the iMac in Target Display Mode?

If it is about the latter, note that the very severe restrictions on Target Display Mode would be likely to make using it impossible - or impractical – even if you did the downgrade. "Nothing later than High Sierra on the iMac" is just one of the restrictions you would need to tick off.