Why cannot I say "tvoje" here?
You can. We accept 498 versions including "Kde je tvoje koupelna?"
zachod, toaleta, koupelna: which one is the best to say when asking my Czech daughter-in-law, "Where is the bathroom?....not to shower or bathe......
koupelna is for bathing, but it may include a toilet bowl, if it is large enough.
Otherwise you want záchod or toaleta (this one may sound posh at home, but not so much in public).