Why can't I De-authorized my Mac ?

I'm having trouble de-authorizing a Macintosh.

It works perfectly, and would like to continue to use it, but we are at our ID limit.

To get to the section where the ID is de-authorized I had to sign in with my Apple ID.

I did the following:

1.) Open iTunes on the computer you want to deauthorize.

2.) Choose Store > Deauthorize This Computer.

After doing this, I received the following pop-up. " This computer was not authorized. To authorize this computer, play a song or video you have purchased using this Apple ID."

This message leads me to believe the computer was never authorized to begin with. I am confused.