Unknown error code during application install "-24" I just updated my Moto X to latest and greatest Android 4.4 KitKat (I even did full factory reset just to be sure) and now I'm trying to install WhatsApp Messenger but unable to do so, I'm getting following error:


  Unknown error code during application install "-24"


Please advise.
According to a thread on AndroidCentral, this is a problem with either...


the app itself having trouble to update an already installed version (unlikely after a factory-reset) → uninstall first, then try a fresh install
certificates not being handled properly → install from a different .apk (e.g. one from your backups, if you've made any, or from an alternative source)
certificates (variant 2) → Go to Settings→Apps, select the "All" tab, scroll to Google services framework, tap the entry, "clear cache", "uninstall updates". Repeat the same for Google Play Store, then reboot, then try again


I didn't verify this (don't have the issue, so I didn't even have the chance to), but it sounds reasonable.

Additional sources on this error:


Unknown error code during application install: "- 24 " (XDA Developers)
App description of the PocketTool App:


  The -24 error is a killer. People think it is because of PocketTool when it is actually the device. You may want to include in the description that if they get the -24 error to reboot the device, then try reinstalling mcpe. If that doesn't work, uninstall all apps that could be used to modify the game, reboot, and try again. If even that doesn't work, the only other option I am aware of is to back up important things such as Kindle books, music, movies, and saved app data on something (I used my laptop) and do a factory reset. If they are on Jelly Bean, then they will get a -24 error if they try to reinstall mcpe after a failed mod install.
  Users had the same trouble with Spotify. A second factory-reset solved it.
  A thread on Android-Hilfe.DE (German) again supports the first variant of my answer (remains of a previous install). Maybe your factory-reset did not complete cleanly.
  A Google search on "google-play error-24" reveals even more sources.