Deleting files during the process probably removed some temporary files needed for the "Go back" feature in Windows 11. This feature relies on having a recent system restore point to revert to Windows 10, and deleting files might have gotten rid of that point.
"Go Back" Option is the easiest method and only works within 10 days of upgrading to Windows 11. It uses system restore points to roll back.
Since "Go back" is no longer available, you'll need to proceed with a clean install of Windows 10 using the ISO you downloaded.