We had a daughter in 2022 and I put her social on my return but didnt claim her, now the mother is trying to claim her but her return is being rejected due to this. Why?

[xmasbaby0]

If all of you are living together as a family then only one parent should claim the child; the other parent should not enter anything about the child on their tax return.   If you used your child's SSN on your tax return, you have put that number into the system and your GF cannot e-file using the child's SSN now.   The only way she can file and claim the child is to file by mail.   When your return has been fully processed, you will need to amend to remove the child from your return.  

[DMarkM1]

It appears she was claimed as a dependent on your return.  In the "My Info" section for your daughter if you indicated she lived with you for 7 months or more then you needed to also answer that you had a agreement with the other parent and that you will not be claiming her.  

Now you can complete the form 8332 relinquishing your claim and give the form to the mother who can mail in her return with your child as her dependent along with the form 8332.  IRS will send you a letter letting you know that someone else has a claim for your dependent.  You will then get a bill for any refund you got that was related to your dependent.  

As you might surmise this will take some time to sort out.