Self Assessment Form Mistake

Hi all, I dont know how I do it, but accidently I make a mistake on my form. I register for self assessment only because a jump over 50k (51650). First time in my life. Week ago they sent me a letter and ask for another 3 years back (19-20, 20-21,21-22). I dont want to wait and start do it ASAP. And I make a mistske in 19-20 form. I put my taken tax in incorrect box. I put it on "tips and other non P60 ernings". I realy dont know how this happend! Because i fill two other form without any issues. Now system seen that i dont pay tax for all 19-20 year. Now system told me i need do pay back 7400 pounds of tax and penalties! And i can't amend that bacause its form 2020 form. I contact call center, they can't change that aswell, they seen fault but can't do nothing. They told me to write a letter what I do today ASAP. I sent it. Whay you guys think? Did they change that? I write all information on letter, where is mistake, where to find it. I attatched my P60 and letter form HM SA. Thay you think? Did letter help or where to find solutions. I dont have 7 grand in my pocket. KC

Hi

You have taken the correct course of action by writing to HMRC, explaining the mistake you made when completing your 2019-20 tax return.
Please note however, that if you miss the deadline for amending a tax return, your letter should specify that you wish to make an Overpayment Relief claim.
Please refer to the following guidance:                          
Self Assessment tax returns

Thank you