How to pay tax without a National Insurance Number

My cousin and his wife have been living and retiring in the UK since 2017. They have never worked in the UK before. Despite his wife being given National Insurance, my cousin hasn't applied for his own NI. Does my 70-year-old cousin have to apply for their own NI to pay taxes on their inheritance from Hong Kong, please?

Hi Tomas Loucky,
They will need to register for self-assessment from the options below, due to their age they do not require a national Insurance number but are ok for self-assessment.
Check how to register for Self Assessment
Number - 0300 200 3310
Overseas - +44 161 931 9070
Open - Monday to Friday 8am to 6pm Closed on weekends and bank holidays.
Thank you.