There are a lot of things that could possibly be going on here, but no matter what, your ex-husband is not going to be able to e-file his tax return this year. He will have to print it, sign it, and file it by mail.
Here a few possibilities, but these are just guesses. And in most of these cases, there's nothing you can do about it. Your ex will just have to file his tax return by mail, claiming your daughter, and let the IRS sort it out.
Look at the Form 1040 that you filed. Is your daughter listed in the Dependents section near the top? If so, somehow you claimed her, even though you didn't intend to. If that's what happened, you should file an amended return to remove your daughter. Otherwise you and your ex-husband will both get letters from the IRS and will have to respond to the letters. But even if you file an amended return, your ex still will not be able to e-file.
If you didn't inadvertently claim your daughter, it sounds like someone other than you and your ex claimed your daughter. Is there any other family member who might have claimed her? Perhaps your parents or your ex-husband's parents? Did anyone else live in the same home with your daughter, or contribute to her support, who might have felt entitled to claim her?
How old is your daughter? Could she have filed a tax return for herself and not indicated that she could be claimed as a dependent?
Could your ex-husband be mistaken? Is he claiming any other dependents besides your daughter. The IRS reject messages are not always clear about which dependent was already claimed. Are there other children in the picture? Could your ex have mistakenly claimed the wrong child?
