Happened with me as well, using IDFC First Bank FASTag on HP Pay.
I uninstalled the app (iOS), reinstalled, logged in, searched for a HP Pay QR code and it's working now.
I had to face this issue twice, in case of refueling four-wheeler, I was able to refuel via DT Plus POS machine by sharing OTP and in case of two-wheeler, had to resort with other mode of payment. I'll keep this in mind going forward and it's irritating for sure, HP Pay app developer(s) should look into this.