Did you claim your child as a dependent on a 2020 tax return? If you did not have enough income earned from working in 2020 you cannot get the child tax credit on your tax return, since getting the CTC on your return does require income earned from working.
That does not mean you cannot get the enhanced child tax credit for 2021. The tax law changed for 2021. If you are not receiving the monthly payments you can still get the credit when you file a 2021 tax return next year. Did you check your eligibility on the IRS site?
This is a brand new system thrown together by the IRS after Congress changed tax laws in the middle of the tax season. There are going to be bugs, glitches and mistakes made and folks will unfortunately have to be patient until they sort it out.
If you do not receive the monthly payments you will be able to get the CTC on your 2021 tax return.
if you filed a 2020 tax return this year, it's been reported that that can delay your payments if IRS is still looking at your tax return.
When IRS is ready, your payments will be larger so that you get the same expected amount in 2021 but spread over fewer payments.