Self Assessment Form won't allow me to pay my Class 2 NI Contributions

I registered for National Insurance payments and self employed status and was sent a unique Taxpayer number. I was employed for a short period in the beginning of the 2021-22 tax year and earned approx £4000. The rest of the year I earned around £2500 on a self employed basis. I have filled out my self assessment form with no issues but when I came to the 'paying class 2 NI contributions' page I was left pretty confused. I had previously selected to use the £1000 trading allowance instead of calculating my expenses so my total taxable income came to around £5500. This therefore meant that no tax or NI was due. However, it said that I had the option to pay voluntary class 2 NI contributions if I earned under the threshold. When I selected to do so it came up with an error message saying I could not as "NI amount was not available or is zero" and that I had to select no. If I pay voluntary contributions outside of my self assessment form it looks like I'll have to pay class 3 with is £15.80 per week instead of £3.05. so obviously it would be good to pay it now. Does anyone know why it won't let me? Thanks in advance.

Hi,

If you have been unable to submit the return and pay the amount by 31/01 you will need to speak to NI directly about paying voluntary class 2 once the return has been submitted.

National Insurance: general enquiries

Thank you.