Android device says "App license could not be verified with Google Play!" when starting the app. Check that your device is connected to the internet. Check that your date/time and time zone settings are accurate. If the time is different to the google play servers by too much then it will cause this error. It is recommended that you let the device synchronise its time from the Internet.
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Power off the device completely (on Apple devices, hold the power button until it comes up with "Slide to Power Off", then use the slider to switch it off). Wait for 10 seconds or so, then power the device back up. Wait for it to completely load and then start the application.

Apple Devices: Verify you have the required iOS version on your device. You can check this by going into Settings->General->About and looking at the "Version" field. It must say iOS 5.0 or higher for Pocket Rummi.

Apple Devices: Check if iTunes suggests upgrading your iOS version. To do this, connect your device to your computer, load iTunes and select your device under the Devices section on the left. The summary screen for your device has a Version section in the middle. An [Update] button will be available if there is a newer version of iOS to install. It is recommended that you try the above tips and backup your device before upgrading your iOS version.