Hi
If you have not yet done so, then it may help to update your iPhone to the latest software (recent updates include important security fixes), starting by making a backup:
Update your iPhone, iPad, or iPod touch - Apple Support
It may then also help to check that you are following the instructions here, including as shown within the demonstration video:
Using Apple Pay in stores, within apps, and on the web - Apple Support
If the issue persists, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your iPhone to be checked and, if required, serviced: