Error Prevention Application.InputBox

If you insist a repeat of the loop you obviously restrict flexibility but in basic terms

ResumeCO:
prompt = "Input the material to be used for unique stencil number " & x & " ('OB', 'Mylar' or 'Mag')"
Caption = "Customer Order Information"
StencilMaterial(x) = Application.InputBox(prompt, Caption, Type:=2)
If Val(StencilMaterial(x)) Or StencilMaterial(x) = "False" Then GoTo ResumeCO