I can't get it to stop saying "B2 is not a valid member of Model", what do I do?

I can't get it to stop saying "B2 is not a valid member of Model", what do I do? I have tried many things, and I can't get it to stop. What is wrong here?

local B2 = script.Parent.Parent:WaitForChild("B2")

function OnTouched(Part)
if Part.Name == 'F1' then
        local mag = Part.Velocity.Magnitude
        print('Returned Result:', mag)
    if mag > 20 and mag < 50 then
            script.Parent.Transparency = 1
            script.Parent.CanCollide = false
            script.Parent.Parent.B2.Transparency = 0
            script.Parent.Parent.B2.CanCollide = true
    end
end
end

script.Parent.Touched:Connect(OnTouched)