I converted this to a comment, since it was posted in the answer box.
So if you ask and answer your own question the odds of someone else giving an answer go way down.
Delete the dependent then enter them again and read all the questions carefully ... one common error is on the support screen ... it asks if the child provided their own support NOT YOU.... if you still are not allowed to claim the child then what is the reason given on this screen ...
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.
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 if you child was born during the year they are considered to have lived with you the Whole Y ear so be sure to answer all year (pick the top box for the Whole Year). You will have to have their SS# before you can file.
Or the one that says did your CHILD pay more than half of their support. It's not asking if you did.
Also you need to enter a ssn for them.
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. People are saying that if you transferred from last year then one of the spouses came over with it checked that you can be claimed. Go to My Info and click Edit by your name or spouse's name and uncheck the 3th box on the second screen, it says Someone else can claim you.
A child born in 2018 (even on Dec 31) is considered to live with you the whole year. So pick the Whole Year at the top of the drop down box.