Hi
It may help to check that they have accepted the payment by following the instructions here:
- Send, receive, and request money with Apple Pay - Apple Support
- If you need help sending or receiving money in Messages with Apple Pay - Apple Support
If you still need help, then I suggest that you contact Apple Support via the link at the bottom of the second support article.
(Note that you are not talking to Apple here - this is a user-based technical support community).