Down-grade from BIg Sur
When trying to install High Sierra on an iMac running Big Sur I get this message "This copy of the "lnstall macOS" application is too old to be opened on this version of macOS”.
iMac 27″, 11.6
Unless you already have High Sierra on a bootable USB installer, you may find it difficult to reinstall the OS.
You'll need to boot the Mac in Recovery mode, then wipe the drive and reinstall the OS that way.
However it is that you choose to reinstall macOS, you will want to have a current Time Machine or other backup of your user files and data.
How to reinstall macOs - Apple Support