Admin password? Is it different than the pw I have been using for a long time now? Don't recall having another special pw?

Hello!

I removed a mac mini from my icloud account so I can sell it, but now it's locked and ask for the admin PW. Tried my apple icloud password & the pw I once used to log on to the mac but it says, "Failed to create activation request." What can I do? Don't recall having another special pw? Never had this happen before I removed the mac mini from icloud. Thx