Some IRS forms are sent in PDF format when you e-File, behind your back. Maybe you did not know that.
This is automatic and is different from explicitly attaching your own PDF.
The issue here is that a PDF is sent with your Form 1040, but when you go to amend with Form 1040-X, TurboTax "can't find it". As a result, you can't e-File the Form 1040-X.
@semicurious
@Mike9241
