If your Self Assessment tax return will not allow you to voluntarily pay class 2 NIC, then it would appear that you are not yet registered as self-employed.
You will need to contact 0300 200 3500 (Monday to Friday: 8am to 6pm Closed on Saturdays, Sundays and Bank Holidays), so that Self Assessment can be updated.
This helpline will also help you make any voluntary class 2 NIC payments.
You can also pay online here:
Pay Class 2 National Insurance if you do not pay through Self Assessment