My very first post here is to inquire about this very same thing.

ProSeries shows 2020 1040X as being Final and ready for EF; yet when I select to EFile the 1040X, the error message states "cannot electronically file Form 1040-X that isn't for calendar year 2020."

Through the prior weeks I've had an opportunity to inquire IRS agents about Efiling 2020 1040X, and they indicate that they have had no guidance for when Efiling 2020 1040X would be available.