I had the same issues today and finally went to Google Flights and booked the Air Asia flight via Google flights and once I had the Air Asia confirmation I went to the AK site and booked seats. Very cumbersome but it worked and very close to same price. Air Asia has to have the worst technology around and they are sabotaging themselves as I was ready to book MH instead just not as convenient. Air Asia you need to drop this OTP requirement or if you have it offer email as a way to get it