Import of movie made in Blender

Export the Blender file as a common file format like .mp4 - save it to your Desktop or somewhere you can find, then import it manually into Unity (dont directly export to your Unity project, to avoid that error - because during export, Blender is actually creating each frame and re-writing/adding it to the incomplete file thats already imported, so if something goes wrong during that time, then itll be corrupt and probably get you that error)

It should be fine once you manually import it.