By default, Maya often only supports .avi (Windows) or .mov (Mac) for playblasts. How to Get an MP4:
Use a tool like FFmpeg or Shutter Encoder to export your MP4 as a series of images.
A "Playblast" is a way to preview your animation without a full render.
If your video won't load, it is likely a missing system-level codec rather than a Maya bug.
