I assume you entered her? There are a couple questions you could have answered wrong under My Info. Click on My Info then click Edit by Dependents, then click Edit by their name and go though all the screens again.
First of all check their date of birth. Also you need to enter a ssn for them. And say her ssn is valid for employment.
If your child was away at college, count the number of months away as having lived with you. The correct answer to claim the person is to say "the whole year". This will indicate you supported the person the whole year, thus it will be entered on your return correctly showing up as a dependent. Or the one that says did your CHILD pay more than half of their support. It's not asking if you did.
If you can't get it to change then try completely deleting them and re-entering them. Sometimes that helps to clear something out.
Or check and make sure you didn't accidentally check the box that says you (or your spouse) can be claimed as a dependent on someone else's return. You can't claim any dependents if you (or your spouse) is a dependent.