Here are possible reasons why.
1. For some unknown reasons' Telkom and 8ta does not work on this code.
2. If you are not using Telkom or 8ta then the phone number that you are accesing the service with is not associated with your SASSA account.
3. The umoya manje service was associated with easypay bank but since SASSA pay will be done through post bank' I suggest you get a cellphone banking USSD code for post bank for pre paid purchasing.