Quicktime wont open file

Using HEIF or HEVC media on Apple devices - Apple Support

My hunch would be that your MOV file has H.265 (HEVC) video, which is supported by Apple apps in High Sierra and later. You can verify this with a tool like Invisor Lite. Even if not H.265, then do post here what codecs are in the MOV. Then we can suggest something different, based on better information.

• The person recording that video could set it to record in AVC (H.264) for future reference, to make it easier on you.
• If this video comes from an iPhone/iPad/iPod Touch, then it could have been converted automatically, when connected by wire and imported into an Apple app like Photos or Image Capture.
• You may need separate 3rd party conversion software, to convert H.265 to H.264, if the previous step can’t be done.