Fill out the return and claim the daughter. You will not be able to e-file since your daughter's social security number will already be used and the IRS will reject is you e-file.
Instead print out the return, sign in ink, add any supporting documents, add a letter explaining that your ex-husband claimed the child and should not have claimed her.
The cover sheet of the printout will have a list of documents to send with the return and the address to mail it. Take it to the post office and send it so that you will have a notice when the IRS gets it.
The IRS will get to your return and determine if you will get the refund and he will have to return the money to the IRS. It will probably take a very long time, possibly a year. The IRS is very far behind in processing returns due to COVID and tax law changes.
