Perhaps the (uncompressed) imported data exceeds you memory capacity. Try importing just a frame or a few frames.
Import["/Users/apple/Downloads/Sample-7.MOV", {"ImageList", 1}]
Import["/Users/apple/Downloads/Sample-7.MOV", {"ImageList", Range[1, 10]}]
Without a reproducible example, it's hard to know if this is exactly the issue or not.