Apple Pay not working

What app? Wallet? What happens when you try? How are you trying to open it? Are you getting any error messages? Have you been able to set it up?

Set up Apple Pay in Wallet on iPhone - Apple Support

Make purchases using Apple Pay - Apple Support

Without more information, that is an impossible question to answer. What is happening when you try to use Apple Pay? Are you getting an error message? If so, what does it say?