If statement doesn't run at all?

Your problem was quite simple, since lua is case-sensitive, you'd have to spell Walkspeed to WalkSpeed.

function Slow(victim, duration, potency)
                print'attempted to apply slow 2'
                if victim and victim.Parent and victim.Parent.Head and victim.Parent.Torso then
                    if victim and victim.Parent and victim.Parent:FindFirstChild('Humanoid') then
                        print'found humanoid'
                        local humanoid = victim.Parent.Humanoid
                        print'debug'
                        if humanoid.WalkSpeed > potency then
                            print'greater than potency'
                            humanoid.WalkSpeed = humanoid.WalkSpeed - potency
                            wait(duration)
                            humanoid.WalkSpeed = humanoid.WalkSpeed + potency
                        else
                            print'less'
                            local original = humanoid.WalkSpeed
                            humanoid.WalkSpeed = 0
                            wait(duration)
                            humanoid.WalkSpeed = humanoid.WalkSpeed + original
                        end
                    end
                end
            end