Issues with PDU Session Modification

Hello,

I’m trying to modify a PDU Session after initializing it for a certain UE with a certain PDU ID. I see that the messages are already implemented (GetUlNasTransport_PduSessionModificationRequest) but when I run my code I get in the Free5GC the following error message:

The following code is the one I’m using, it is based on the one from PDU Session Establishment:

    sNssai := models.Snssai{
		Sst: 1,
		Sd:  "010203",
	}
	//ueSupi := strings.Split(ue.Supi, "-") [1]
	//supiInt, _ := strconv.Atoi(ueSupi)
	//
	//pduId := int64(supiInt % 1e4)

	pdu = nasTestpacket.GetUlNasTransport_PduSessionModificationRequest(pduId, nasMessage.ULNASTransportRequestTypeExistingPduSession, "internet", &sNssai)
	pdu, _ = tglib.EncodeNasPduWithSecurity(ue, pdu, nas.SecurityHeaderTypeIntegrityProtectedAndCiphered, true, false)
	sendMsg, _ = tglib.GetUplinkNASTransport(ue.AmfUeNgapId, ue.RanUeNgapId, pdu)
	_, _ = conn.Write(sendMsg)

	fmt.Println("HALAAAAAAAA")


	// receive 12. NGAP-PDU Session Resource Setup Request(DL nas transport((NAS msg-PDU session setup Accept)))
	n, _ = conn.Read(recvMsg)
	_, _ = ngap.Decoder(recvMsg[:n])

	fmt.Println("oleeeeee")


	// send 14. NGAP-PDU Session Resource Setup Response
	sendMsg, _ = tglib.GetPDUSessionResourceSetupResponse(ue.AmfUeNgapId, ue.RanUeNgapId, int64(pduId), GNBG) //pduId HAS BEEN ADDED HERE AND IN PACKET.GO AND BUILD.GO
	_, _ = conn.Write(sendMsg)



	return pdu

Does anybody have an idea of what the problem is ? ANy help would be very apreciate it.