For anyone who stumbles across this post, I phoned HMRC and the reason why "Self Assessment" was not appearing in my account is because my UTR number had been associated with a temporary NI number. I have no idea why. They merged the numbers together and now I can do self assessment filings online.
I received a letter on 24 November 2022 asking if I needed to pay the High Income Child Benefit Charge for 2021 to 2022.
I have been earning more than 50k since August 2020 and was not aware of the rules so I registered for self-assessment and received a letter on January 2023 with my UTR code. My Wife - who receives the child benefit - also cancelled the benefit.
Nothing ever appeared in my personal tax account which I checked semi-regularly.
On 6th of April, I received a letter saying it was time to fill in my tax return for April 2022 to April 2023.
Again, nothing in my personal tax account relating to self assessment.
On 26th of April, I received 3 letters saying I owe £100 penalty (x3) for not sending my tax returns for years ending April 2020, April 2021, and April 2022.
Still nothing in my personal tax account relating to self assessment.
So now I owe £300 and still cannot figure out how to pay what I owe. I'd love to pay what I owe - which now has an extra £300 fee on top - but I cannot figure out how.