HMRC doesn't recognise my NI number

I tried to register for self assessment, and it asked to check my identity (to enter my full name, DOB, and my NI number). However after I entered these details, it returned: Unable to verify your identity Sorry, there is a problem with the service Try again later. I have the NI number on the payslip from my previous job, so this cannot be wrong. I have tried multiple times, same result. I also tried "forgot NI number" and it asked me to enter my postcode, but same result as well. I have not changed address since I got my NI number. Before this, I already verified myself using GOV.UK Verify (uploaded passport, driving license etc). Is there anything else I can do? Or who should I contact to address this issue?

Hi oscarli,

Please try the following., 

•    clear your cache
•    try another browser
•    check you have entered your details correctly and in the correct format

If unsuccessful

•    if the documents you are using are correct,
•    select ‘get help with this page’ or 'Is this page not working properly' 
•    report the issue with our Digital Customer Support Team (DCST)
•    until you can pass IV, you cannot continue with the online journey to view their PTA / BTA 
•    you will need to contact the relevant Line of Business

Thank you.
 

I have tried countless times to put my details in and they aren’t recognised. Just realised that it wont let me input the last letter of my NI so it’s then incorrect.

Hi Sobby2711 OBrien,

If your details are not accepted, you will need to contact us directly. Self Assessment: general enquiries

Thank you.