It is working for me, so maybe one of these thoughts will help:

1. I assume Line 6a is over $2500, right?
2. Have you gone through the error-check/review to clear all errors?
3. Is there anything semi-unsual with the return, such as ITINs?
4. Have you verified the child is showing up as a dependent on the 1040?
5. Child was under age 17 at the end of 2020 (check the birthdate)?