If there is no email present in the Quickbooks Online vendor record, NO eCheck will be created or sent. A email must be added to the vendor record for the payment you're creating in Quickbooks to become an eCheck.
First, from the far left menu, choose "Expenses". Then, at the top of page, click on "Vendors"
Click on the name of the vendor that needs an email.
In the upper right corner click the "Edit" button
In the Vendor Information, type in the recipient's email address and in the lower right corner click "Save"
Once the system notices the email has been added and the date is correct, it will automatically email out the eCheck.