What is the flash sync speed?
Flash won’t work at very fast shutter speeds because of the way your camera works
You can’t use flash across the entire range of shutter speeds. This is because the focal plane shutters used in D-SLRs achieve higher shutter speeds by exposing the sensor through a narrow slit between the shutter blinds, and any flash would only hit a sliver of sensor. The maximum flash sync speed you can use depends on the camera model and is typically 1/200 or 1/250 sec.