I just unlocked a Mann Co. Supply crate key and got a "Classic strangifier", which apparently changes normal items to strange items. However, when I click "Use with...", it says that I have no eligible items to use with the strangifier. I've been playing for a while and have plenty of weapons. How can I use this?
The strangifier is used on a weapon called "The Classic". It's a primary weapon for the sniper. When used on the weapon it turns it into a strange Classic which starts counting kills for it. You probably just don't have a classic in your inventory to use it on.