See @Critter-3 's explanation of how the tax benefits may be split, in your situation. The other parent e-filed first and most likely claimed something he is not allowed (e.g. Earned Income Credit). This prevents you from e-filing. You will have to mail in your return.
He probably didn't know about the splitting rules and will now have to file an amended return. But that cannot happen fast enough to allow you to e-file. The IRS will process your paper return and send you your refund even if he hasn't amended yet.
Ref: https://www.irs.gov/publications/p17#en_US_2017_publink1000170897
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