Child benfit - need to pay for previous years

Hello, I recently realised that due to an error with my reading of my P60 form I needed to pay back some child benefit for the years 2021-22 and 2022-23. I have registered for self assessment and now have a UTR. However I cannot work out how to file a self assessment for those years and how large any possible penalties would be. It is a genuine error that I am declaring and seeking to rectify but I am only being offered the chance to file a return for the year 2023-24. All help would be greatly appreciated. Thanks.

Hi Paul Fletcher,
We would need to check that tax returns have been issued for the earlier tax years which would then allow you to file the tax returns online.
If you contact HMRC we can check this for you:
Self Assessment: general enquiries
Thank you. 

Hi,
We would need to register you for Self Assessment for those years now passed.
Contact us by webchat or phone via:
Self Assessment: general enquiries to allow us to do this, and give you any further advice necessary.
Thank you.