I filed an amended return in February, and also had the same notice in September.
One question: have you moved since the address you filed with? I found out a number of things, in my case:
1) The IRS will send you a check, not direct deposit. (They are direct depositing the child credit this year, but didn't use that route)
2) They will send the check to the address you filed under. (In my case, 2018, even though they know my current address)
3) The USPS will not forward a federal check, even if you have a forwarding instruction. The check is returned to the IRS.
In my case, I gave them until early October, then called following the instructions below. I got a person in about 5 minutes. They were able to update my address, and get a new check issued. I got an automated letter by the end of October, explaining the issue with check and giving instructions to fix, but of course I had already taken care of it.
https://amynorthardcpa.com/how-do-i-reach-a-real-person-at-the-irs/So, you should have received a letter by now, I would think.