I have a Asus zenfone 2 running official android m. I had the same problem as the guy in this thread: Home Screen and Recent Apps buttons don't work
Before I could fix it, I tried the guest mode method. I pressed on the guest mode icon in the quick settings, it switched to guest mode, stayed there for about 5 seconds, then automatically switched back to my own profile and suddenly the users icon disappeared from the quick settings tray and the users option disappeared from the settings page. No more possible ways to switch users or enter guest mode.
I solved the main problem using the nova launcher method, but my phone didn't have the setup wizard test activity, instead I managed to fix it with the Asus setup, not the stock Android one.
It fixed everything, but the users setting is still missing. I've ran the check max-users command in the terminal and it said 1. Phone isn't rooted so I can't change that. Is there any way for me to get back the users or guest mode option without rooting?