The following could be the possible causes
1. You have not associated a Gmail account with your Android powered AG device - this is esssential to using the Google Play Store. You will have to create an account (if you do not have it at: https://accounts.google.com/signup
and use the credentials thereof to login to the Play Store upon its prompts.
2. Just like any other app used on Android devices like your, the Play Store relies of cache and this cache get corrupt at times. Please note that this will only apply if you have associated a Gmail/Google account with your AG device. To clear this possibly corrupt cache from the Play Store, go to Settings > App > All > Play Store > Clearcache... and confirm.
Retry the Play Store app and observe. Kindly comment below if you still have issues on this.