Faulty formulation: Because provision and one's outcome are decreed, effort, prayer, lawful means and righteous action are unnecessary.
Correct formulation: Belief in divine decree does not abolish the use of means. While entirely subject to Allah's knowledge and will, a person remains responsible for acting. Supplication, lawful work and worship belong to that responsibility.
Application: An ill person may seek treatment; a student studies; a believer prays and works toward a good end. Taking these means does not challenge destiny. It is part of fulfilling one's duties.