To create a free trial for subscriptions, edit or create a product that's set for reoccurring payment.

You can find your products by visiting the 'access' section on your global App Settings or any page that requires purchase when you click 'add product' with the 'require purchase' toggle turned on. You get to determine the length of the free trial too (any number of days).

Note: you will not see the Free Trial option it the product is set for 'One off' payment.