@tetsuwanatomu I had the exact same frustrations and experience. I tried to amend my 2020 return, and TurboTax gives all indications that it will file the amended return electronically, until the very last step at which point it says unsuccessful/unable to transmit. And to confuse matters more, I called TurboTax customer support twice, and was told both times that I COULD e-file my amended return. So Intuit's own employees don't know how it works.
To add insult to injury, my problem was caused by TurboTax not properly calculating differences in mortgage interest deductions between Fed & State - a basic, simple thing. The software also completely failed in accounting for a mortgage I refinanced after the 2017 tax law revision.
The amount of paper and ink I wasted and the amount of time I've put into this is absurd.
