We filed my daughters tax return.she checked that someone else could claim her as a dependent, but my return is being rejected because it's saying we can't claim her.

Print out her tax return, and look at the 1040 to see what is actually marked on that form....just under the filing status is a box  for "Someone can claim you as a dependent"

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No matter what the situation...if any of your kids are filing, always make sure you file your own first. 

That avoids mistakes about that situation and gets the most important one in to the IRS first.