Sometimes the child files their first return and forgets to mark that they can be claimed. It could be the other parent claiming the child, it can be a mistake - those still happen. You will have to mail your return and write a letter explaining that you tried to e-file but could not. Explain your position with any relevant information. Do use tracking to ensure the safe arrival of your information.
References:
When Both Parents Claim a Child
Dependent Dilemmas
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