Go back to My Info on the left side of the online program screen. Edit the dependent and carefully check your answers to the questions.
Make sure you select them as your Child, that you selected that they lived in your home for the Whole Year, temporary absence for education is deemed living in your home. That they did NOT provide over one-half of their own support (You Did), that you indicated there was no one else in the household providing support to them, and that you did NOT check the box labeled Not valid for employment in the Social Security number section.
If all your answers are correct then delete them as a dependent and re-enter.