I can't update WhatsApp, Snapchat and many other apps in my system, that is 9.0.1.
I tried to change date and time, turn off my iPhone and sign out.
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According to Apple Support System Status, at the time of writing 2015/10/13, the iTunes Store Service may be unavailable for some users.
Try updating to iOS 9.0.2. It's all bug fixes and it might fix your problem.