I understand on your 2020 tax return the Credit for other dependent box is checked. You will want to amend your 2020 tax return.
From Tax Home, click on Amend my Return. Then select update other information on my return, and you will see your My Info. You will click Edit or the circle with the dots to the right of your youngest dependent that needs to be edited.
You will then go through the interview questions for that dependent. Check your answers for year of date of birth and also on the screen with the social security number, make sure the box is not marked for not eligible for employment.
Once you have gone through the questions, you should receive the screen that you qualify for the Child Tax Credit.
The amended returns do take up to 16 weeks for the IRS to process. There is an IRS tool to update dependent information to receive the advance payments for the 2021 Child Tax Credit. The IRS expects to add this feature to their tool by late summer.
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
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