Often, if your driver receives a "Decode Error" it is due to one of several things:
The volume is to low, and must be turned up with the buttons on the right side of the tablet
The driver swiped the card incorrectly, The card should be swiped quickly and firmly pushed into the card reader to make sure there is a good read. A majority of decode errors are due to incorrect swipes.
Once a driver receives a Decode Error, they must hit the purchase button and start the process over again.
Below is an example of a decode error caused by a card being swiped to slowly: