Apple Pay is part of the iPhone OS and firmware/hardware of your iPhone. It is automatically updated when the iOS is updated.
If you’ve confirmed the above, please update your iPhone to the latest iOS, 17.3 or higher. Older iPhones may only support 15.7.5, but Apple Pay will function.
Update iOS on iPhone - Apple Support
Once updated, please do a forced restart of your iPhone.
Force restart iPhone - Apple Support
Make sure your are signed into your Apple ID/iCloud account on your iPhone.
Sign out of iCloud on your iPhone, iPad, Apple TV, or Mac - Apple Support
Follow the steps in this support article for adding credit/debit cards to your Wallet.
Set up Apple Pay - Apple Support