... make sure you contact the National Insurance Helpline to sort it out asap. We didn't and it got us in a pickle which has taken a while to sort out....
My son got a casual job when he turned 16, and needed his NIN for the payroll. He didn't have the letter confirming it, but I'd read on Mumsnet that the NIN is the same number as the Child Trust Fund reference, so he gave his employer that. All seemed fine. But ...
When he turned 18, he got a Lifetime ISA and deposited some birthday money. The provider tried to claim his 25% government bonus, but the claim was rejected due to a "mismatch of personal details" with HMRC. We had lots of back and forth with the provider re-confirming his name, date of birth, address, etc. He also installed the HMRC app to confirm his details there. He put in his personal details, and the app confirmed his NIN was the one he was using. So ...
The LISA provider claimed the bonus again, but it failed again. Once more, he re-confirmed to them that his details were correct. However now when he checked the HMRC app he was unable to get into the "Personal Details" section. Unhelpfully, it said "Service not Available - please try later". What it should have said is: "There is a problem with your NIN, please contact the National Insurance Helpline". Eventually we did this. It took an hour to get through, as well as ingenuity and resilience to get past the voice-bot (which was much more certain than we were that we could find the answer online by visiting their website). When we finally got through, and explained the issue, it was sorted out in 5 minutes - we were told we had the right NIN but the NI account hadn't been fully set up - that is why my son never got the letter that should have arrived shortly before his 16th birthday. We think the reason it wasn't fully set up was because we never claimed child benefit for him, but I guess we'll never know. 🤷♀️
Lesson learned for us - so I thought I'd park it here in case it helps others in future.