If my childs dad listed but did not claim one of our children as a dependent, why am I getting an error for claiming that child as my depend. We were each claiming one

The dad put those Social Security numbers into the system in such a way that you cannot e-file this year.  SSN rejections cannot be fixed.  The only way for you to file and claim the child is to file by mail.   Print, sign and date your return and mail it in.  Sadly, it will take a lot longer to be processed.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.

And.....the child's father will eventually need to amend his return to remove the child he was not supposed to claim.  But you do not need to wait for that.  Go ahead and mail your return.   He can amend later when his return has been processed.