You have reached the 50 phone lifetime cap on buying directly from Net10/Tracfone. The only way around it is with a different person/address/card, and that person would start deducting from their 50 phone cap. You will likely have to buy at a store from now on -- no reasonable way around it.
Once, Ian Littman used several people's addresses/cards for this and Tracfone figured it out, they went so far as to block his whole city from internet purchases! Did you happen to move to Texas recently (I forgot the city, it was a small one)?