Due to a bug in Android, WhatsApp could not be installed in Android Runtime (ART) on Android 4.4. The issue with ART has been fixed in Android (4.4.1). Please update your OS to at least 4.4.1 to use WhatsApp with ART.
If you are unable to update to 4.4.1, please use Dalvik instead of ART. This is an issue with the way that ART verifies apps, not with WhatsApp. WhatsApp will still function properly in Dalvik.
If you attempted to re-install WhatsApp in ART, your
/data/data/ directory may have a latent com.whatsapp folder left behind from the failed install that will prevent you from installing WhatsApp again. Android does not have any user exposed function to resolve this issue. Currently the only known way to remove this file is to either delete the file (only possible if your phone is rooted), or to reset your device to factory defaults.
A bug has been reported to Google regarding this issue, see here for other possible solutions and more information.