Hi
Follow the instructions here, if necessary including contacting Apple Support via the link in the second article:
- 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
Payments are credited to your Apple Pay Cash Card balance and can be transferred from there to your bank account if desired.
When receiving money via Apple Pay Cash, funds may be available immediately, but account security checks might require additional time.