How do I fix this issue with my gun not syncing movement with my arms?

[rArticuno]

@PostVivic

GunWeld.C0 = GunWeld.C0:Lerp(CFrame.new(0, 0.5, 0) * CFrame.Angles(-math.asin((mouse.Origin.p - mouse.Hit.p).unit.y) - Vector3.new(0, 0.5, 0), 0, 0), 0.1)

I got this error: