I have both the game and privacy settings are all public. Why isn't it allowing me to link them then?
make sure this is how your steam profile look you can revert these changes once linked https://imgur.com/m008i0y
Do you own the game "Supreme Commander: Forged Alliance" or do you own a different game?
If you own "Supreme Commander 2" or "Supreme Commander" that doesn't work.
You don't have to buy a bundle. The only game you need is "Supreme Commander: Forged Alliance"