Alright. I changed selectedWeapon = player:WaitForChild("StarterPack"):FindFirstChild("Knife")
to:
selectedWeapon = player:WaitForChild("Backpack"):FindFirstChild("Knife")
Now i can press the key 3 and it chose the Knife. Thats good. But if i press again the Key 3, the Knife isnt more in my Backpack. But i still have it in my hands.
Also if i press key 1 (Because i want to chose the Primary gun), it gives me the Knife and in the Backpack changed the Name of the Knife. Maybe i can send a Video