Conventionally, when Students click 'Send Email' to send their Report as an email, it Commits the Lesson. The 'Send Email' button becomes 'Email Sent' and is no longer clickable.
However, if you need the Students to re-send the Report as an Email (for example you've put in the wrong recipient email address to receive the Reports), you can do so by 'Granting Extensions' to all the Students (Student Work > Lesson of Interest > Manage Student Progress > Click 'Grant Extension to All'):
Doing this undoes the Commit of the lesson, and re-enables the 'Send Email' button.
Alternatively, you may click an individual Student within the Manage Student Progress menu, and Grant them an individual extension by clicking the Grant Extension. This is a more efficient approach if only one or two student emails are missing.