You can add the child as a dependent on the amended 2021 tax return. However, the child will not be eligible for the Child Tax Credit since the SSN was issued after the 2021 tax return due date. You may be able to receive the Other Dependent tax credit of $500, if eligible, for the child.
See this TurboTax support FAQ for amending a 2021 tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-2021-return/L1gEU4G3X_U...
