While in the return, and only once the return has been completed, find the Diagnostics button and click it to check the return for any issues. If any diagnostic errors are present, they will be shown in red and clickable. Clicking these errors will take you to the issue in most cases.
You may click on the Create E-File button at the bottom of the Diagnostics window if there are no errors. After a short period, you will receive a notification in the top-left corner informing you that your E-File is Created.
Afterward, click on the Send E-Files tab, and you will find your created e-files listed here and ready for sending.