Please make sure you are only typing the date in using numbers - no special characters or spaces. There should be 8 numbers and the format to use is DD/MM/YYYY. Ex: If your birthday is April 5 1954, then you would type: 05041954
You may want to select and delete everything in each box before retyping, just in case there is a hidden space causing the problem.
FYI: If the dependant was born in 2021, then you would not include that child on your 2020 tax return. You can include them on your 2021 tax return.