I, too, was told by TurboTax support that you could no longer e-file a 1040-X for 2019 using TurboTax (even though the IRS says e-filing is available for 2019 and 2020 amended returns).
If you end up mailing your 1040-X please google "Where to File Form 1040-X" or "Instructions for Form 1040-X." The mailing addresses for people living in Alabama, Alaska, Arkansas, California, Georgia, Hawaii, Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio, Oklahoma, Washington, and Wisconsin have changed beginning Calendar Year 2021.
The address shown on my File by Mail instructions from TurboTax for my 2019 amended return is no longer correct.
