Am just downloading Xcode 10.1 Beta 3. I am going to try it to see if it will enable me to load my appon to my new iPhone running iOS 12.1.
I can easily transfer the app using Xcode 10.0 with Xcode 9.4.1 command line tools to my iPad running iOS 12.0
When I try the same thing with my iPhone running 12.1, it won't work. I am hoping that Xcode 10.1 will support iOS 12.1 and allow me to transfer the app from my MacBook to my iPhone.
I installed Xcode 10.1 beta 3. I installed the additional command line tools when it offered.
I went to Xcode-->Preferences-->Locations and set the environment to use Xcode 9.4.1 command line tools
I opened Window--> Devices and Simulators. I saw my iPhone there and right clicked on the listing to make sure that it had a valid provisioning profile (I had installed one previously, but if it doesn't;t have one, that is the next step)
I then copied my app on to the phone and it worked seamlessly.
(I am still using High Sierra on my macBook)
Hope this helps anyone having trouble with this