See PHI1990. A customer in the EU reports that their card provider is refusing charges which do not use "SecureCode". This appears to be a Mastercard marketing name for a technology known as "3D Secure 2".

Stripe supports this technology, the Phortune integration just currently does not.

In theory, this ("Strong Customer Authentication", "SCA") is broadly required (by "PSD2") after December 31, 2020. In practice, it's unclear how widely this is actually enforced, since EU customers mostly should have processed at least one bill after December 31, 2020 by now.

This is likely not terribly difficult to implement technically, it just seems to require wading through a lot of acronyms and process language.