I got the quest at one point after traveling with Cait to heal her of her addictions, yet it never popped up in my quest screen. When I went to the vault and she sat in the chair I went to the console to activate it and it said error no test subject detected, any way to fix this? I play on xbox one btw.

Do you have a recent save before she sits in the chair?

In my game (PC) I explored Vault 95 with MacCready, when I found the clinic Cait teleported from Starlight Drive-In to the clinic. Kinda weird, but okay... had no problems completing her quest.

I also experienced the same problem on my xbox one. I have tried everything and nothing seems to make it work. Also when walking in the vault i somehow got warped into the sky and fell to my death.