Hi,
From your screenshot, it seems that the file big_buck_bunny.mp4 could not be found in the media folder defined by config.lua. Please edit config.lua so that applications > rootDirectory > mediaStorage > mediaFolder points to the folder where you store big_buck_bunny.mp4 and other media files. For example, a typical setting is
mediaFolder="..media"
which is relative to the current location. An absolute path, for example