My childs other parent claimed my son last year. We have a divorce decree saying is was my year. How can I claim him now? For 2021 She received the money in March.

[xmasbaby0]

You forgot to mention which parent the child lives with, and that is what the IRS cares about.  As far as the IRS is concerned, the custodial parent is the one who can claim the child.   Which parent did the child spend the most nights with?  Is there a signed form 8332 that allows the non-custodial parent to get the child tax credit?

Are you the custodial parent?  Do you have an agreement with the other parent to allow the other parent to claim them--due to divorce or that you live apart and share custody?  Did one of you sign a Form 8332?

If there is a signed 8332 then the custodial parent retains the right to file as Head of Household, get earned income credit and the childcare credit.  The non-custodial parent gets the child tax credit for children under the age of 18.

As far as the IRS is concerned, the custodial parent is the one with whom the child spent the most nights during the tax year--at least 183 nights.

If you are a non-married couple who live together then only one of you can claim the child(ren) and the one not claiming the child does not enter anything at all on their tax return about the child. 

If someone who should not/could not claim your child filed a tax return that claimed the child, then you have to prepare your own return claiming the child and MAIL your tax return to the IRS.   The IRS will sort out who can claim the child.   Eventually both of you will receive letters from the IRS and will need to show the IRS who the child lived with.

[Critter-3]

First make sure what you are eligible to claim ... if you are the non custodial parent you only get the dependent exemption and the CTC ... everything else belongs to the custodial parent.  So did your return get rejected because you also tried to get the EIC ?   You can paper file to claim the child now even if they already have been claimed.   The IRS will then question both of you and you will both need to address an IRS notice in a couple of years.

There is no such thing in the Federal tax law as 50/50, split, or joint custody.  The IRS only recognizes physical custody (which parent the child lived with the greater part, but over half, of the tax year.  That parent is the custodial parent; the other parent is the noncustodial parent.)

Who can claim the exemption and credits depends on who is the custodial parent. (By the IRS definition of custodial parent for tax purposes - this is not the same as the legal custody that a court might grant.).

The test that the IRS uses to determine the custodial parent is where the child lived for more than 1/2 (or greater part) of the year. The IRS will go so far as to require counting the nights spend in each household - that person is the custodial parent for tax purposes (if exactly equal and more than 183 days - The custodial parent is the parent with the highest AGI, if less than 183 days then neither parent has custody so the child cannot be claimed by either parent). And yes they are that picky.

See Custodial parent and noncustodial parent  under the residency test in Pub 17

https://www.irs.gov/publications/p17#en_US_2017_publink1000170899
 
Only the Custodial parent can claim: (Child would be listed as non-dependent EIC & CC only)
-Head of Household 
-Earned Income Credit
-Child Care Credit

The non custodial parent can only claim: (Child would be listed as dependent)
-The Exemption
-The Child Tax Credit

But only if specifically specified in a pre-2009 divorce decree, separation agreement or the custodial spouse releases the exemption with a signed 8332 form - after 2009 the IRS only accepts a signed 8332 form that must be attached to the non-custodial parents tax return.