Looks like Apple broke something on their mobile app store in a recent iOS update.
Devices stuck on old iOS builds… specifically iOS 3.1.3 are currently unable to purchase or download apps from the Apple App Store.
They are able to update their existing apps. They are also able to install apps through iTunes. But the app store on the device itself is no longer working for them.
Apple is yet to respond to these complaints. We assume this is a technical glitch that would be sorted out soon enough.
Apple is not allowing some of the older versions of iPhone and iPod Touch to update to the recent iOS editions. This includes the first generation iPhone and the first and second generation iPod Touch models.