You may have paid the child benefit tax charge via your tax code in 19/20 and that's why there is nothing to pay.
Please ensure you have included your income and tax on the return and check your self assessment statement via your online account to check if you owe anything.
However, if you're still unsure you may need to contact us to check this for you on our helpline or via webchat:
0300 200 3310
Self Assessment: webchat