If you were rejected with code F1040-506-01, indicating that some else has claimed your child, then you need to ask your relatives who might have claimed the child. There are tie-breaker rules governing who has the higher right (see below). If you have the higher right, then you should ask the culprit to file an amended return, omitting your child.
https://www.eitc.irs.gov/EITC-Central/abouteitc/basic-qualifications/tiebreaker
If none of your relatives or friends claimed the child, then you may be a victim of identity theft. If this is the case, you should immediately file form 14039. See more information below.
https://turbotax.intuit.com/tax-tools/tax-tips/General-Tax-Tips/Identity-Theft--What-to-Do-if-Someon...
To get your return accepted by the IRS, you will need to file by mail. If you prepared Form 14039, you should include a copy of the form plus the identification documents you included when you initially notified the IRS. You will be required to produce documents to support your claim to the dependent, but do not send these until requested by the IRS.
