What if my return is rejected because other parent claimed my son when divorce decree says i claim him and i forgot to claim more dependants? How do i fix this?

To fix this, you can go back into your return under Personal Info, and go through dependents to enter whatever dependents you need to add.  If you have a dependent claimed by someone else, you can leave them on the return and file on paper by mail.  The IRS will send you your refund and will come back at some point in the next year and ask each of you for evidence that you had the right to claim the child.  Whoever loses will have to pay tax plus interest.

I do want to tell you that, despite your divorce decree, the parent with whom the child spent the most nights during the year has the right to claim the child.  That parent must give the other parent an 8332, which passes the right to claim the child to the other parent.  Th other parent can then claim the dependent exemption, and the child tax credit.  The Earned Income Credit and Child Care credit stays with the parent with whom the child spent the most nights.