WhatsApp not removable on some devices

When WazzapMigrator completes its work you need to reinstall WhatsApp, in order to let it import the Windows Phone messages converted by WazzapMigrator. Some devices (like Samsung S6 or S7) comes with WhatsApp pre-installed and you can't uninstall it. In this case you just need to proceed like this from Android settings:

Settings -> Apps -> WhatsApp -> Force stop + Clear data

On Android 6 (Marshmallow) devices (ie. Samsung S7) it's a little bit different: