You can always go into a canter from a trot...
Maybe what she meant was posting trot.
Because you should not go into canter from posting trot...but instead from sitting trot.
Usually it takes good training from the horse to be able to canter from a walk..and the rider must be advance and very specific and with her aids. You need to be able to use independent aids unitedly very fluently and without confusion to the horse.
Same goes for canter from a halt though this is more advanced still.
So really, its fine to go into the canter from a trot. Most riders do when hacking or pleasure riding. Usually they go from a collected trot to a canter in most general training too. Transitions of every type have their own purpose and benefits!!