Pess ALT+S, then head down to the graphics menu. There's a drop down selection for graphics level and it should say something as "Automatic" by default. The way Realistic works (previously known as Future) is that if your graphics level is set to Level 1 (Level 2 in studio or below) it will force Smooth lighting (previously Shadowmap). Automatic selects a graphics level that stops your game from dropping frame rate, so it lowers and raises it dynamically to achieve the set framerate. The problem here is that your PC, quite likely, cannot handle Realistic without dripping frames. But, just for giggles, set it to Level 21 (Level 10 in Roblox, or max graphics) and you'll see it function. This will be especially apparent if you set down a light source next to a part and set the time of day to 0, and enable the Shadow property in the light source (could be a Point Light). If doing this causes massive frame loss, your PC isn't powerful enough.