error while sending AT command for USSD

I have been trying to check my credit in sim card by using the at commands but it has not been fruitful. I have been trying to solve this for a while now but nothing is working. Please, i need help.
Below is the list of command and their respective responses:

AT+CMGF=1

OK

AT+CSCS=“GSM”

OK

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

OK

+CME ERROR: 264