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
