The reason your return was rejected because one of your dependents was already claimed as a dependent on another return. The way to tell which dependent is causing the reject, there is a description in the review field with a a number in brackets, [?]. For example: "[2]" will identify the second dependent from the top.
Go back through your return and verify that the dependent's information has been entered accurately. Correct any misspellings, make sure the dependent SSN is entered correctly. If there was an error, fix it and file the return.
If everything is correct If you want to electronically file this return and claim the dependent, an Identity Protection PIN will be required. Visit IRS IP PIN for further information.