See this TurboTax help article:
Can I e-file my federal Form 1040-X amended return? (intuit.com)
Note that, if your original return was paper-filed, then your amended file must also be paper-filed.
Note also that the help article applies to federal returns only. The rules for an amended state return may differ.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
