Option to use Apple Pay not showing up on United Airlines app

I am trying to use Apple Pay as my payment method to purchase a ticket on the United Airlines app. However, when I go to payment method, the option to use Apple Pay is not there. My friend purchased their ticket through the United app, and they had the option to use Apple Pay. I'm not sure why this is happening, because whenever I shop using other apps, I always have the option to use Apple Pay with no problem. I also contacted Apple Support, but they didn't have a solution for me.

Both my iPhone and United Airlines app are up-to-date. I've tried deleting and redownloading the app, and I've tried restarting my phone -- neither have resolved the problem.