The main difference between a turboprop and a turbofan is the speed of sound. A propeller needs to avoid breaking the speed of sound, otherwise you get obscene

The main difference between a turboprop and a turbofan is the speed of sound. A propeller needs to avoid breaking the speed of sound, otherwise you get obscene noise, a reduction in efficiency, and potentially damage to the propeller (although there are some rare propellers that do regularly break the sound barrier). This means that propeller-driven planes must have a speed limit that is quite a lot lower than the speed of sound, to avoid the propeller tips breaking that speed barrier. A turbofan however is typically installed on planes that want to fly around 80% of the speed of sound. That's really close to the limit, and there isn't enough leeway there for a propeller to get away with it. A turbofan manages by using lots of blades to more gently push the air back, and then the cowling helps by preventing blade-tip vortices, and by constricting the airflow slightly, which increases the air's backwards speed. So a propeller and a turbofan are suitable for different plane speed regimes. When you're travelling slowly, it is most efficient to push a very large amount of air back just a little bit faster than the plane is moving, and a propeller is great at doing that. As you approach the speed of sound, that becomes infeasible, and a turbofan becomes more efficient. If you're trying to break the speed barrier, then you need a lower bypass ratio, or you may even need to switch to a turbojet.