"Non sono certo che lei prenda l'autobus".
DL: I am not certain that she takes the bus.
Is it deemed too colloquial to use a very common English expression "........she gets the bus"?
I would think so. For all my lessons, I go out of my way to avoid all English colloquialisms, even if they are extremely common. I try to remember that this a machine I'm interacting with and unlike us humans, machines have their limitations. ;-)
You can try to "report it" the next time you get the sentence in a lesson, and choose "My answer should be accepted", when given the chance but, yes-- I would say it is too idiomatic for most users and the admins.
Otherwise they'd also have to accept "catches the bus", or even "makes the bus".