what do you mean he claimed her earlier this year? on his 2020 return or did he use the IRS portal to register because he'll be claiming her in 2021. only the one claiming her on their 2021 return gets the CTC.
If the other parent claimed the child on their 2020 tax return but you plan to claim the child on your 2021 return, then:
The IRS says they will eventually update their web site to allow you to tell them this fact and enroll in the advance payments. But it is not ready yet. Remember this is a tax credit on your 2021 return. If you can't enroll or don't get the full amount of the advance payment, you will get the credit added to your tax refund. The advance payment doesn't change how much you get, it only changes when you get paid.
Also, the child's other parent should probably go to the IRS web site and unenroll from the advance payments. If they don't claim the child, they will have to pay back the advance payments when they file their tax return.