GST REFUND APPLICATION

we had given some service to SEZ and charged GST but customer did not pay us GST saying that it is exempted. in the GSTR 1 we had shown the same
under supply to SEZ with payment of tax and in the GSTR 3B ,we had shown the same under normal outward supply. while selecting the form for refund under supply to SEZ with payment of tax it gives a message - No outward supply during that period. please inform as to how we have to file refund application