On another note, their system will check to see what country your IP address is coming from when completing the application and payment. So you have to be in the country of application either physically or virtually (VPN)
Also could be an internet browser issue of not popping up a window required for transaction. Try using a computer or tablet. I had this issue and had to switch from iPhone to a tablet and different browser. It went right through upon the change.
I just use the bill pay part of my bank app on my phone. I saved it as a favorite and it will store the account number and check the balance and fill in the amount to pay.