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@JoeP78 ,
Great question. I'm sorry that this  happened to you. Unfortunately, it does happen too often.
There are tie breaker rules that the IRS uses to determine which parent should claim the child on their tax return.

Sometimes a child meets the rules to be a qualifying child of more than one person. The following rules must be applied to determine who can claim the child as a qualifying child. Under the tie-breaker rule, the child is treated as a qualifying child:

• The parent, if only one of the persons is the child's parent,
• The parent with whom the child lived the longest during the tax year, if two of the persons are the child's parent and they do not file a joint return together,
• The parent with the highest AGI if the child lived with each parent for the same amount of time during the tax year, and they do not file a joint return together,
• A non-parent, if no parent claims the child as a qualifying child although he or she may do so and only if the non-parent's AGI is higher than the highest AGI of any parent who may claim the child, or
• The person with the highest AGI, if none of the persons is the child's parent.

This information is found in Publication 501, Dependents, Standard Deduction, and Filing Information.

If your return was rejected, you may want to paper file your return and submit paperwork and evidence showing that you are the one who should be able to claim the child. 
It is up to the IRS to decide if the divorce agreement should be used as a reason for you to claim the child.

I would highly suggest paper filing the return and submit evidence as to why you are the one that is able to claim the dependent.
The IRS will then get involved and may request more information from the both of you in order to decide who should be able to claim the dependent.

Feel free to review the following IRS website for more information about claiming a dependent:
https://www.irs.gov/newsroom/claiming-a-child-as-a-dependent-when-parents-are-divorced-separated-or-...

I hope the issue gets settled quickly.