Missing 1040 from 2019

The IRS sent me a letter stating they didn’t receive my 2019 tax return. I have a PDF copy of my return, which was completed using TurboTax, and it includes an Electronic Postmark - Certificate of Electronic Filing dated 4/16/2020. How can I obtain more information to prove that I submitted this return? It seems the IRS doesn't have a record of it. Who can I contact for help?

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