Please make sure you are running the latest version of Kindle for Android, which is 8.0. If your device is running an earlier Android OS version, you will have the option of downloading the latest compatible version of Kindle for Android, from the Google Play Store.
Hey @Phone: As the Amazon Staff member suggested above, the current Kindle for Android works for Android 8.0
On the Google playstore you will need to look for the options to see if they have a Kindle for Android 5.1
I'm sorry, but I am not Amazon and do not have and Android so can not check on the Google play store for you if there is a version compatible with 5.1 - it may be that that old version of Android can not support it any more :(