I have a late 2012 Imac but purchase it from someone on craiglist after I did clean instaĺl of high sierra it upgrade to catalina now I have a lock on my efi but have no receipt Iguess Iam screwed

Since you were able to perform a clean install of macOS it would seem the firmware was not locked when you purchased the iMac.

If you disabled this Mac using FindMy, then you can use the instructions in this Apple article to unlock it:

If your Mac is lost or stolen - Apple Support

Otherwise it sounds like this iMac might be still managed by a previous owner who has "disabled" this Mac which will also enable the firmware lock. If you don't know the firmware password or code, then you will not be able to disable it. Apple will only remove a firmware lock if you can provide proper proof of purchase and ID which you won't be able to do unless you purchased a refurb unit directly from Apple (or maybe an authorized Apple reseller).

This is one of the dangers of buying a used Mac unless you buy it directly through Apple.