Hard Copy Key Wont Work On Steam

Hello SC players and fans. I've recently got back into supreme commander and went ahead and downloaded FAF client where i noticed that my physical disk key could be used as a Steam code, in my library, and immediately attempted to do so. Only to find out that according to Steam the code is invalid. I've retyped key in 3 types carefully making sure it's right caps and all to no avail.
I'm posting this because i don't know what to do about this now. I don't mind having to rebuy the game but would rather not. If anyone has had a similar issue and resolved it please contact me; and I thank you in advance.

It's not exactly a Steam code. You don't enter it in the same place as you would put a "Steam code"

Under the "Games" tab at the top you select "Activate a Product on Steam..."