Transak requires iOS users to be on their latest version (iOS 16+) in order to use Apple Pay as the payment method.
How to update your iOS version:
1) Go to your mobile Settings > General > Software Update
2) The screen shows the currently installed version of iOS and whether an update is available, if so at the bottom you'll see the option to download the latest iOS version.
Note: Make sure you have enough storage to install the update