Daycare

Only the custodial parent can claim the child care credit. This is true even if the custodial parent gives the other parent permission to claim the dependency via a signed form 8332. The IRS doesn’t involve itself or care about what is contained in a divorce agreement. 

1. First, only the parent who had actual physical custody more than half the nights of the year can claim the child as a dependent.  If the non-custodial parent is supposed to claim the child, the custodial parent must give the non-custodial parent a signed form 8332 dependent release.
2. The IRS goes by actual physical custody, counting nights if needed, not by what a court order says about custody. 
3. If you have been alternating claiming the child without form 8332, you have been doing it wrong.  The custodial parent may be giving up certain tax benefits and the non-custodial parent may be claiming unallowed benefits.
4. Even if you exchange the dependent using form 8332, the child care credit can only by claimed by the parent where the child lives more than half the nights of the year. That benefit can't be split, shared, or transferred.
5. I don't know what you have done in the past so I can't say what might be different.
6. If you had custody more than half the year and your tax return is rejected, it is because your ex said the child lived with him more than half the year, and is not using (or you did not give him) form 8332.  Print and sign your return and file by mail, and the IRS should send letters to both of you about the discrepancy in 9-12 months.  If you did not have custody but you are trying to claim your child, you need form 8332.  Answer in Turbotax that you did not have custody more than half the year, and indicate that yes, you have form 8332.  You must also remove the day care credit.  After e-filing, you must mail the signed original form 8332 to the IRS within 3 days, using form 8453 as a cover page.