DEAUTHORIZE DEVICE

See this document: https://support.apple.com/en-us/HT204385

Specifically the section on deauthorizing ALL of your computers.

Assuming you're referring to deauthorizing iTunes. If you mean something else, you have to explain what you're talking about.

You haven't really provided enough information to do anything other than guess.

So you couldn't perform the actions that did not require you having the computer? Such as:

"Sign out of iTunes

Open iTunes. From the menu bar at the top of your computer screen or at the top of the iTunes window, choose

Account > Authorizations > Deauthorize This Computer. When prompted, enter your Apple ID and password. Then click Deauthorize. Learn more about deauthorizing your computer using iTunes, including how to deauthorize all the computers you've used with your iTunes account."

What to do before you sell, give away, or trade in your Mac - Apple Support