Every time I go to Steam, it always says "The action is only valid for products that are currently installed." I encounter this error whenever I try to uninstall.
So are you asking how to install or get the products valid?
You are trying to perform an action on a promo item that was available for a limited amount of time. If you can't uninstall the item from Steam, go to Control Panel and use Add or Remove programs. Alternatively, the item will disappear automatically when you restart Steam if the offer expired.