First, you will want to ensure that the amount is set to perfectly match your bill for the first transaction. To verify this, click on the name of the bill qube, tap the three dots in the top-right corner, select "Settings", then check that the amount listed next to "Bill Amount" perfectly matches the initial transaction amount.
From there, make sure your qube is funded. To check this, click on the name of the qube and make sure that the "Balance" amount is equal to or more than the "Bill Amount."
If this bill qube has worked before, please pause then unpause it, and try the bill processing again. To do this click on the name of the qube, tap the three dots in the top-right corner, then select "Pause Payment."