Hi there! I understand you're having trouble completing a payment when booking a flight using the Allegiant app on your iPhone. Let's work through this together.

To get started, can you tell me if you’re seeing any specific error message when the payment fails? Also, have you tried using a different payment method or card? Sometimes, payment issues can be related to the card being declined or flagged for security reasons.

Here’s a simple step-by-step we can try first:

1. Double-check your payment information in the app: make sure the card number, expiration date, CVV, and billing address are entered correctly.
2. Ensure your card has sufficient funds and no holds or restrictions set by your bank.
3. Try restarting your iPhone to clear any temporary glitches.
4. Log out of the Allegiant app, then log back in and try booking again.
5. If possible, try using a different payment card or method to see if that works instead.

If the app still won’t process payment after these steps, there might be an issue on Allegiant’s side such as a temporary payment processing error or security block on your card. Sometimes, multiple attempts can trigger fraud protection.

If you want, I can guide you through clearing the app cache or reinstalling the app, which can sometimes fix payment glitches on iOS 18.7.

Could you let me know if you’ve tried any of these steps already or if you have any specific error details? That way I can tailor the next steps for you.

Looking forward to helping you get this booked!