Why I can't change direction of my AC from Front to windshield?

First I would try blowing out the air ducts to make sure there is no debris which can block the doors, but if there is no debris the problem is usually that the blend door actuator is bad. That is what controls the direction of the air flow and I have seen them crack, the shaft that actually moves the doors breaks, or the electronics in them can be faulty if applicable.

Replace that and it should fix your issue.