You can amend your 2020 return to check that other box then the child will qualify.
Please see the TurboTax Help article How to amend (change or correct) a return you already filed for instruction on how to amend your previously-filed tax return.
When you file a Form 1040-X, it becomes your new tax return for the tax year in which you file it. It changes the original return to include your new information.
Go into My Info section and click on that dependents name and run through the questions again. Make sure the age is correct and that the box is checked.
Was that child being claimed by the custodial parent ? Did they live with you less than 7 months?
When the IRS fully processes your original return AND the amendment process has opened later this month you can amend the return to capture that credit IF you have to make the correction.