Why does Steam say "requires ownership of another product before activation"?

This error means the key you're trying to activate is for DLC, an expansion, or a season pass — and the base game is not owned on the same Steam account. Steam requires you to own the base game directly on your account before DLC can be activated. Common causes include activating a DLC key before the base game key (check your order for multiple codes), accessing the base game via Steam Family Sharing (this does not count as ownership), or owning the base game on a different Steam account. Make sure the base game shows "In Library" on its Steam store page on the account you're activating the DLC on.