Check list:
Did you have funds in your 'prepaid card'?
Did you have 3G/4G/Wifi connection?
Was the machine that takes the card, 'paypass friendly'?
Did you or the cashier press the "credit" button?
Did you 'tap' the back of your phone (where the battery/antennae) resides on the machine?
Did you tell the cashier "Ok, I'm trying out this new app, so bear with me okay? Let me see if it works, I should be able to pay it with my phone."?
-If you said it above, you shouldn't look like a moron. Do you not have people skills? Did you just try it out to look special and important, and failed miserably?
What did the error say?