I have custody. My ex claimed my child one my year what can I do?

If you had actual physical custody in your home more than half the nights of the year, and you did not give the other parent permission using a signed form 8332 (or a similar letter containing the same information) then only you can legally claim the child as a dependent.  Print, sign and mail your return.  You may want to use a mailing service with tracking and proof of delivery since the IRS is a mess.  In about a year, the IRS will catch up and will send letters to both parents asking for proof of where the children actually lived.  Assuming you prove your case, the other parent will have to repay their tax benefits with interest and penalties.

I would not e-file without the child then file an amended return to claim the child, because amended returns have been taking up to a year to process.

Be aware that when the IRS asks for proof, your court order will carry little weight.  The IRS wants to know where the child actually lived regardless of the court order.  They like proof from outsiders showing where the child lived, like letters from the school or doctors about appointments and so on.  Your child's bus schedule showing they get picked up at your house every day.  Things like that.  Social medial posts and photos showing the child living in your home may also help.