My iPhone X is running iOS 12 beta code. I just noticed the other day that I cannot turn on personal hotspot on the phone. Under Settings/Cellular there is a blue option for Set Up Personal Hotspot, but when selected it sends me to a now defunct web page for Verizon. When I use a browser and login to Verizon, I see that my plan includes mobile hotspot on capable devices...
I'm not sure what I am missing or if this is a known issue with iOS 12 beta.