Possible reasons are as follows:
1, the mobile phone signal is not good, you can connect the signal is good WIFI, or replace the 4G signal and then pay.
2, separate change or the money in the bank card, are not enough for this payment, in this case, you can put the money in the bank card into the change, and then pay.
Maybe the Wi-Fi is not working, or the signal is not working, so the payment fails