To give your customers the option to book themselves into a recurring class series, click:

Classes > Schedule > locate the class you'd like to allow this for > click the dropdown menu to the right of it > Edit Class Template.

Scroll to the bottom > click More Settings > Allow customers to book themselves into the entire series

Click Save and you'll be prompted to choose to update all classes in the series or just classes after a certain date.

Self serving customers will now see this option at checkout.