... I wasn't in the UK tax system before I moved here in 2015.
The test shouldn't be when you were given the NI number ('in the tax system'), but when you were eligible for it. For example, if you were born a UK citizen and left the UK aged 8 and returned to the UK in 2015, I'd expect you to be able to make voluntary contributions back to 2006. However, it sounds like you didn't have UK citizenship when you moved here in 2015, in which case I would only expect them to allow voluntary contributions back to 2015.
I (and others) may have read you posts as you moving
back to the UK in 2015 rather than
to the UK for the first time. Sorry if that was the case.
By the way, some unrelated good news: WEP has just been repealed and we no longer need to consider it for anything. In my case, my (predicted) pension will be 8.42% more :-)