Complex iMac erase

Hey guys, an employee has added her Apple ID to our business Mac, turned her 'Find My Phone' on, and left the company a few months later, uncontactable now!

Now we want to delete our iMac and sell it, but upon trying to erase back to factory settings it is asking us to enter in her password before it will let us do the final erasure (which we now may never get off her).

We always had admin privileges so I don't know how she has managed to do this...is there anyway we can erase the iMac back to factory defaults so we can sell the bloody thing lol!