Apple Pay not working on iPhone 12 Pro Max but works on Apple Watch

Please contact Apple Support and request remote diagnostics be performed. Pay particular attention to the NFC module. If the NFC module is confirmed working, they’ll probably have you erase your iPhone and setup as new.

If you’re in the US please call 1-800-MY-APPLE or outside the US, consult the support articles below.

Contact Apple for support and service - Apple Support

Try resetting network settings.

Reset your network settings

If you're using iOS or iPadOS 15 or later, tap Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings.

If you're using iOS or iPadOS 14 or earlier, tap Settings > General > Reset > Reset Network Settings.

This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.

Do you have many cards in Apple Wallet? What country are you in? Do you use a VPN, ever had VPN software or VPN profile on your iPhone?

When you try an Apple Pay transaction what is the exact error message?