Self Assessment Tax Return Error from 2020_21

I made an error in my tax return on 2020_2021 but only realised this in July this year. It was too late to alter the return online, so I had to write a letter. I wrote this letter with full details and sent it to the relevant address on July 2023, but I have had no reply and I can see that my tax return remains unaltered to account for my error. What should I do now, please anyone??

Hi RachaelD Dando,

You can expect a reply by approximately 22/11/23.

Thank you.