The attendee email field is one of three form fields (the other two are Firstname and Lastname) that appear in every registration form. It's not a mandatory field by default. The good news is that it's really easy to make it mandatory.
To begin, let's just clarify which field we are referring to! The screenshot below shows the field has been set as mandatory. The * tells us this.
1. From the New Event screen
Click on New Event and scroll down the page. Tick the box to make the field mandatory.
2. If you've already set up your event and need to make the field mandatory
Go to your event menu and click Edit Details
Tick the box to make the Attendee Email field mandatory