Lots of people have been getting the Error 918 when trying to update the RunPee app. Here’s something that works for most people.
Go to Settings > Apps > All > Google Play Store > Clear Cache & Clear Data
then go to
Settings > Accounts & Sync > *your Google account* > click menu (the three dots) > Remove Google Account
then re-add your Google Account again. That should do the trick.
If that doesn’t fix it then the problem may be that there isn’t enough room remaining on your internal devise storage to install/update the app. You can try to free up space by moving other applications to the external SD card, assuming you have one. Not all apps can be moved but it’s a good practice to move all the apps you can to the external SD card.
To move an app to the external SD card open settings > More > Application manager.
Select any application and you should see a button labeled “Move to SD card”. If the app can’t be moved then it will be grayed out.
Or, you can just ignore the update and wait for the next one. I’m not sure why the Google Play store has this issue sometimes but it happens to a lot of people with a lot of different apps. Google could certainly do a better job of explaining to the user exactly what’s going on.