A child born in 2021 does not qualify you, retroactively for the 2020 Child Tax credit (CTC). The child does qualify you for the expanded ($3600) 2021 CTC.
There was some talk of the IRS making the advance monthly payments available for parents of 2021 babies, but so far that hasn't happened. See this IRS portal for updates https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
You'll have to wait until you file your 2021 tax return, in 2022.
