Same here: iTunes 12.8 doesn't recognize my iPhone X or iPad Pro 10.5 that are both on iOS 12 PB2. I've tried on my iMac and MacBook Pro.
[doublepost=1531690101][/doublepost]I tried to restore my iPhone X to iOS 11 a couple of times last, but I kept getting 12 on there since it wouldn't connect to iTunes to restore the archived copy. Looks like I'll have to wipe the iPhone, set up as New, and then restore from the iOS 11 backup. I hope I've got that right.