apply the Self Assessment without NI number?

My mother moved to the UK from 2021. She is 71 years old. She hasn't work in the UK and she got the interests from the fixed team saving. Now I would like to help her to apply the self assessment without NI number. What can I do for her because she cannot apply the NI number? Can I help her to do the tax return without NI number? Many thanks.

Hi,
It is possible to register for Self Assessment without a National Insurance number. Details on how to do this can be found here:
Check how to register for Self Assessment
However, you need a National Insurance number to make sure your NI Contributions and tax are recorded against your name only. If you do not have an National Insurance number you can apply for one here:
Apply for a National Insurance number
If you have lost your National Insurance number you can use the guidance here to find it:
Find your National Insurance number
Thank you.

Hello, I have also the same problem. When applying for an NI number for my retired mother, it only allows if the NI is for looking for work and 'You cannot apply for a National Insurance number for any other reason'. Could you help to clarify how we can go through the NI application process? Thank you!

Hi Chan,
You have not said why your mother needs a NI number.
I assume you have tried to use this link to apply – 
Apply for a National Insurance number and because you have presumably stated ‘not working or looking for work’
The response states you cannot use this service to apply. It does not say you cannot apply - 
https://www.apply-national-insurance-number.service.gov.uk/apply/may-not-need-nin.
It goes onto say – 
If you were told to apply for a National Insurance number to get benefits or a student loan paid, you should not use this service. We will contact you and tell you how to apply for a National Insurance number.
If you entered your mobile number as part of your application for benefits or a student loan, we will send you a text telling you what to do. If you did not provide your mobile number, we will send you a letter.
You cannot apply for a National Insurance number for any other reason. For example, you cannot apply for a National Insurance number only because you need to open a bank account, open an Individual Savings Account (ISA) or pay into a pension.
If you have been told you need a National Insurance number for any other reason, contact the organisation that told you this. They should have an alternative process for people who do not have a National Insurance number.
Thank you.