I faced the same issue during e-filing but I was able to find the reason this occurs in the TurboTax Desktop:
1. Click on "Forms" tab
2. Check your all your 1099-B forms
3. In these 1099-B forms, check the last line which has the title "PDF Attachment"
4. In this, the line would read "Taxpayer must mail in a statement with more details?..." -> the "Yes" option will be selected.
There are 2 options to overcome this:
Option 1: This might be particular to my case but one of my 1099-B was incorrectly imported and I fixed the 1099-B entry for this such that there is no additional attachment necessary.
Option 2: Need to send the returns via physical mail with the required attachment details of statements which reflect the 1099-B details.
