I had this happen with one newborn dependent about 10 years ago. I called the IRS Efile Help Desk, and the lady told me it's a glitch in the IRS database with the child's last name. She told me to enter the last name twice. It worked, and we've been doing that every year since. The parents are supposed to get it fixed, but they never have so we just keep duplicating the little girl's last name in the efiled return.

I haven't had to call the IRS Efile Help Desk in a few years. But they used to (a) answer the phone, and (b) be really helpful.