IRS claims my reported 2019 tax return info does not match with what a 3rd party reported (they do match). Can you tell me next steps on how to clear this up? Thank you.

is it the 1099-G the IRS questions or the reported amount of self-employment income or something else? we can't see the notice. check carefully. 

if it was the 1099-G, write back and include a copy of the 1099-G, an explanatory letter saying that the 1099-G enclosed matches the reported amount (sign and date) and a copy of the notice. 

or for a fee (if you haven't already paid for it when you filed)

The Audit Defense service is provided by TaxResources, Inc., also called TaxAudit.com, in partnership with TurboTax. If you paid for Audit Defense and you received an IRS notice, call TaxResources, Inc. at 877-829-9695, or report your IRS notice on their website at http://intuit.taxaudit.com/. Do not contact the IRS until you have spoken to TaxResources. They may contact the IRS on your behalf.