How to change shutter speed on manual mode when movie setting is on?
If your shutter speed would be slower than 1/50, you wouldn't be able to make a movie.

A movie is a sequence of (typically) 24 or 30 pictures ('frames') per second; so to be able to make 30 pictures, each picture can only take 1/30 of a second. Considering that the camera needs to do some processing and refreshing of the receptors, 1/50 is just enough time to make those 30 shots in second.

You can check if your camera has a time-lapse setting, which means you take less than 30 frames per second, then you might be able to set a slower shutter speed. However, the result might be unattractive because any moving objects would be smeared.