They believe that stress is a trigger for schizophrenia so perhaps your body is not healthy enough to take on the stress of exercise.
I realize the medications they give you are horrible but schizophrenia is also horrible and sadly, the horrors go unrecognized by the sufferer. Do not stop the medications. But also don't fall into the typical schizophrenic self-treatment of endless pots of coffee, sugar sandwiches, 7 packs a day of smoking and sleeping during the day and staying up all night. Exercise on top of any of these, let alone all of them, isn't going to do you any favors. Get your circadian rhythm and nutrition under control first.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 180 x 2. Current Deadlift: 230 x 2