"Unknown error code during application install (921)."
Thanks to XDA , there's a thread with the error codes and what they mean, along with ways to fix each issue. Here's a sample of that list with some of the most common error codes:
Solution: This issue usually comes up on devices with multiple accounts. Make sure you're accessing the Google Play Store from the correct account. Next, uninstall the app you were trying to update, then re-download from the correct account.
Solution: Remove your current Google account, re-add the account, and then clear data from the Google Services app (Settings > Application Manager > Google Services).
Solution: Delete the data from both the Google Play and Google Services Framework apps (Settings > Application Manager > Google Play and Google Services). Next, remove your Google account, restart the device, and then re-add your Google account.
Solution: Your device has run out of storage space. You can clean your cache, remove unused apps, or delete photos and videos.
Solution: Delete the cache from the Google Play app (Settings > Application Manager > Google Play). If you are still seeing an error, delete the data from the Google Play app (Settings > Application Manager > Google Services). Finally, if issues persist, delete your Google account and then re-add it.