I need help. My tax return was rejected saying my child was claimed on another form. This is not possible as I've seen my ex wifes proof that she hasn't claimed him.

Not sure what happened.  But if your return is being rejected and the rejection says your child's SSN has been used, the only way you can file is to print, sign and mail your 2019 return and let the IRS sort it out.  You will nto be able to e-file. Sorry.

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.

hate  to say this but what she showed you and what she file could be completely different..  there's nothing you can do about this.   if you have proof she's violating the divorce decree see your attorney.