Self assessment for BNO Visa holder over 70

My mom is now over 70 , she has been retired so she didn’t apply a NI number . The only income is saving interest and government Pensions from HK. She try to complete the self assessment but fail, letter from HMRC stated that she must have NI number. Please advise the next step of action, thank you.

Hi Fiona Tse,
The pension from Hong Kong is only taxable there.
She will only need a return for the savings interest and will therefore need to apply manually for the UTR
Registering for Self Assessment and getting a tax return
Thank you.