Nokia Lumia can't access windows store. Keep getting error message.

Purchased a Nokia Lumia for my wife at Wal-Mart. Even though it was being sold as an ATT GO phone, the tech assured us it was compatible with straight talk (our current service provider). We purchased it and he set everything up for us.

Everything works fine (text, internet, calls) but whenever she tries to access the windows store on the phone, it won't let her log in. Just giver her some kind of error message.

Any ideas?