You will need to wait until TurboTax finalizes the Form 1040X before you can access the online return and respond to the IRS notice 0012C.
IRS Form 1040X, Amended U.S. Individual Income Tax Return is estimated to be available on 03/07/19
See this TurboTax support FAQ for IRS forms availability - https://ttlc.intuit.com/questions/1908854-irs-forms-availability-table-for-turbotax-individual-perso...
See this TurboTax support FAQ for a Form 8962 and 1095-A for the IRS notice 0012C - https://ttlc.intuit.com/questions/4348755-the-irs-is-requesting-form-8962-for-2018
