If you go to Supporters > Volunteers In Training and press the blue Training Bookings button you should see a list of all those volunteers that still require at least one module of training.
The list will show each module's status for that volunteer, whether booked, not booked or completed.
From here you can either click on the person to get their invite link, or you can click the blue envelope circle. This will create an Outlook email to the person prepopulated with their details but also editable so you can personalise before sending.