A child born in 2021 cannot be entered as a dependent on a 2020 tax return. You can add her as a dependent on the 2021 tax return to be filed next year.
The IRS will be updating their portal sometime this year so that you can add a child born in 2021 for the Advance Child Tax Credit. Go to this IRS website (Manage Payments) - https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
When (or if) that option on the portal opens you can add the child ... if it never happens you will just get it on the 2021 return when you file next year. READ Q F3 ... https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...