QuickTime Player Not Working When Trying to Open Files
Most Mac users are happy using the basic programs pre-installed on the operating system, such as Music, Movie, and OneDrive. But those who expect greater functionality from their media players might find that, occasionally, their QuickTime Player won’t work when trying to open different file types.
"Older or specialized media formats might not work in your app, because the format requires software designed to support it. If that happens, your app won't open the file or won't play back its audio or video." — Official developer Apple
One of the major benefits of Apple’s patented media player, QuickTime, is that it’s free and supports basic video file types: .mov, .mp4, .4mpv, MPEG-1, MPEG-2, 3GPP2, 3GPP, DV, AVI, and AVCHD video formats, plus audio files such as MP3, .m4a, .m4b, .m4p, AIFF, .caf, AMR, WAV, AU, SD2, SND.
With such a range of accepted file types, it’s surprising and frustrating when QuickTime can’t open a file. There are several reasons for this error and below we will consider each of the reasons in detail.