Child Benefit penalty

Hello all My daughter was born in the 21 22 financial year. We claimed child benefit as me and partner earned under the 50k threshold (each). It is my understanding you don't complete a return if you both earn under 50k. In 22 23, I earned more thanks to a claim for unpaid wages, and ny earnings went to 57k. In November I completed a self assessment and was stated to owe 700 quid. I paid this on 2nd January. Today I have received a letter from hmrc acknowledging this payment but giving me a penalty of 100 pounds for "late filing of 2021 22 return" They apparently notified me on the Web portal on 12th March but never emailed, wrote or texted. The due date was 18th April. How can they make me pay a penalty for a return I didn't have to file as I earned under the threshold? And now it's more than 30 days since the notification have I lost my right to appeal?

Hi CLewis91,
If you'd like to appeal the penalty, you can do so here.
Appeal a Self Assessment penalty for late filing or late payment.
Alternatively, write to us confirming why you disagree with the penalty notification.
Please  send your appeal to Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom.
Thank you.