3D secure is a fantastic technology that protects buyers and sellers and shifts the liability on payment disputes (a great thing, if you've ever been a victim of credit card fraud!). However, one of the side-effects of requiring 3D secure is its impact on subscriptions.
As 3D Secure requires a step from the owner to authenticate a charge on the 3D Secure card, this means that subscriptions on cards with required 3D Secure validation will automatically fail. By default, this prevents new subscriptions from being created on cards that require 3D Secure validation. However, this default behaviour can be changed by toggling on "Allow Incomplete Payments on Subscriptions" under Configure > Payments in the form editor.
This setting removes the requirement for the first charge to immediately succeed, which allows 3D secure payments to work.
Note that when this setting is enabled, it may mean a subscription is created in Stripe before the submitter has successfully finished being charged.