AT&T sim card was not able to register in mobile network, prepaid

Purchased a new SIM from store. Before buying it, AT&T store guy assured that prepaid SIM will work for the given IMEI. He checked the compatibility. But after payment, when cell phone powered up with the new SIM, AT&T sim card is not able to register in mobile network, prepaid.

Then store guy suggested to buy the new cell phone from the store. It seems, i am trapped with the selling strategy and feeling cheated.

This SIM is of no use for me and want my money back.