“+CME ERROR: unknown network message” error while sending AT USSD command

I am trying to execute USSD codes through my GSM modem. When i try USSD command which gives me some options, it is executed but i can't get any operation done from the options.

AT+CMGF=1

OK AT+CSCS="GSM"

OK

AT+CUSD=1,"*101#",15 (To check balance)

OK

+CME ERROR: 264

AT+CUSD=1,"*17108#",15 (To get a data pack)

OK

CUSD: 1,"1.Datapacks .Voicepacks .SMS Packs .Billing info ote: Back:* Top:#",15

AT+CUSD=1,"1"

OK

+CME ERROR: 264

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