If you bought the phone for Tracfone, then it has to be used with Tracfone talk time. In order to use it with Straight Talk, you would have had to buy it from Straight Talk's website or a Walmart location.
Straight Talk is owned by Tracfone, but they are separate brands so unfortunately phones won't work on the opposite plans.
You should call a Straight Talk representative and double check or see what you should do.
Here's the official Straight Talk blog: http://www.straighttalkblog.com/