I've been trying to test an app on an iOS 12 device, but every time I build it, it successfully builds on the device (my iPhone or iPad) but crashes and I receive this message:
Unable to launch /private/var/containers/Bundle/Application/
(I'm using Xcode 10 beta 3 and Swift 4.2)

1- Check if you are using distribution profile while running the application directly from Xcode to device, if so please use developer profile instead.
2- Check if your device iOS is higher than the beta 3 as you are using Xcode10 beta3. If higher, try to use same Xcode beta version as your iOS
If thats not the case provide more info about the Build setting you are using.