My Mac and my old iPhone 6 used to work with personal hotspot just fine.
I replaced my old iPhone 6 with an iPhone 7 and transferred all my apps and data from the 6 to the 7. Most things worked fine, but not Hotspot.
My Mac remembers my old iPhone 6 and remembers the Hotspot password for that old connection. I think that my Mac thinks the new iPhone 7 is the old iPhone 6 because they have the same name. The Mac shows the Personal Hotspot item in the Wifi menu, where it used to work. When I select the item, the Mac tries to connect using the old password. Now, if I remembered the old password, I could change it on the new iPhone 7 side, but I don't remember the old password.
I can see the password on the iPhone 7, and I want to input that password on the Mac. But I can't find the place on the Mac where I can delete the old password, or change it, or delete the Hotspot item and have it try again from scratch. It just keeps pounding away and telling me "failed to enable Personal Hotspot…"
I tried Personal Hotspot through WiFi. I tried Personal Hotspot through Bluetooth. I tried rebooting.
I just want to find the data on the Mac side and change or delete it (I think if I delete it then the Mac will try again from scratch and prompt me for my password).
EDIT: I found the iPhone entry under the Network -> Advanced -> Wifi gadget and deleted it. Still "failed to enable."
Best Answer
I took the pair of the Mac and the iPhone 7+ to the Apple store. I demonstrated the problem, we rebooted both machines, and demonstrated the problem persisting.
The troubleshooter gave the phone a "Vulcan death pinch" squeezing the lower volume key on the left simultaneously with the usual power key on the right, holding them until the phone fully rebooted to the Apple icon on the screen, releasing only then. He said it's a hard reset, and different on different iPhone model numbers. After that, we toggled the Personal Hotspot button in the Settings app on the iPhone, making sure it's on, and chose the Personal Hotspot from the wifi gadget on the upper right of the top menu bar of the Mac and the Mac asked for the Personal Hotspot password, which I copied off the phone.
To repeat and emphasize, we hard-reset the iPhone without changing the Mac. We didn't have to do anything extraordinary on the Mac, not even open the System Preferences Network dialog.