The truth is that word order isn't actually too important in Finnish. Both work perfectly, both are correct. "Missä on voi?" / "Missä voi on?" are the most common since they roll on the tongue nicely. "Voi on missä?" is like you are being a bit scatterbrain, but still perfectly understandable And if you conjugate word "on" a bit you can even say "onko voi missä?" All those means exactly the same thing, and there isn't much difference in nuance either. Textbooks are bad for the reason that they usually only teach one way to say things, so don't get stuck on them.