E-filing is available for 2019 F.1040X: https://www.irs.gov/forms-pubs/new-information-for-form-1040-x-filers

Intuit did tell us way back when this was first rolled out that none of their professional tax products would support returns that were previously e-filed with third party products.  We asked a few moderators for the rationale here but they never responded.

According to ProSeries' instructions for 1040X, from TY2020 onwards, you're supposed to be able to check a box within the program to verify the amended return can be e-filed if the original was e-filed with a third party product: (see step 4 under To e-file the amended return) https://proconnect.intuit.com/community/amend-tax-return/help/how-do-i-amend-an-individual-return-us...

However, others have complained that function doesn't really work and were told by Intuit Support that they released the instructions prematurely.  It was put to a vote (which normally takes 20 years or more to complete) and there's been no update about the status of that suggestion since: https://proconnect.intuit.com/community/proseries-tax-idea-exchange/1040x-e-filing/idi-p/98753/page/...

If you can't e-file the 2019 F.1040X, it's probably because it was e-filed with a third party software or that it's paper filed.