Are you asking about getting the child tax credit on your 2020 tax return or are you asking about getting the monthly advance payments? Please clarify----and tell us how old your child(ren) will be at the end of 2021. Did you file a 2020 tax return? Has it been fully processed?
According to the IRS if your return, even an amended one is still processing then you will not receive the advance child tax credit payments. Go to topic Q B 12 as of June 30th there were 35 million US income tax returns that had not been processed and estimated only 3% of phone calls were being answered. Verify that here