Try this info to see if it helps. I would say the video is playing on the cloud system and being streamed to your local PC. When you try to download it, the iCloud system goes into an anti piracy mode so no video content can be taken. There are a couple things to try: To check to see if the video has actually been transferred to your PC use the video player named VLC Media Player to try playing it. Also try software named "Handbrake" which is free and open the program to see what it says about video/codecs. A MP4 file can have just audio and not video.
https://www.coolmuster.com/ios-recovery/download-videos-from-icloud.html