The IRS sent me a notice CP81 stating I didn't file my 2021 return. I have a confirmation from TurboTax that it was accepted on March 13, 2022. How do I resolve this?

You will have to mail a signed copy of the Form 1040, including all forms and schedules.  Also, copies of W-2 and Form 1099 which includes taxes withheld.  Include with the mailed return your proof of filing, either the TurboTax email showing the date the IRS Accepted the return and/or a copy of the Electronic Postmark Report which has the date/time the return was transmitted and the date it was accepted by the IRS.

Mail the tax return to the address shown on the notice you received.

Go to this IRS website for the CP81 notice - https://www.irs.gov/individuals/understanding-your-cp81-notice

Even though you have proof that it was accepted, if the IRS lost the file then your only option is to print and mail the return.