Welcome to the Community @indigosky! 

When a client saves their card information online, because you require prepayment or you've enabled no-show protection, that card information is not saved as a card on file.

If you'd like to save a client's card on file for future Appointments you can add their card information to the online Appointments Calendar (when creating an appointment for them) or to their customer profile from your online Customer Directory (no appointment needed).

I do see how it would be efficient to be able to give clients the option to save their card information during online booking! We'll let you know in this thread if this feature is available in future.