Unable to add Self Assessment to Business Account

Hello, I tried multiple times but I am unable to add in the Business Account section of the HMRC website my Self Assessment data. Has anything changed? I am trying to re-add it by using my UTR and NI number and I get a message saying that what I have inserted does not match what HMRC holds - this is wrong as I tried multiple times and 100% sure details are correct. What can I do? I used to have access to that until maybe a month ago, but they have now disappeared.

Hi,

When trying to add the service, enter your UTR and postcode instead of your National Insurance number as there may be an issue with the online system.

Thank you.

Thanks, I spoke with HMRC call handler and they cannot see it either. Not even my accountant. This apparently has to do with my account being put on hold because you sent me a letter about a repayment which I had to reply to.