There can be several reasons for it, like codec issues, software issues, filename inaccuracy, and more. It can be discouraging to find out that QuickTime Player cannot open video files on your Mac. QuickTime Player Makes Video Files Not Open on Mac
So you need to resolve all the internal Mac errors and make sure to download reliable antivirus software that helps you scan all the issues and get rid of the virus that infected your computer. For instance, if your computer is infected with malware, it may prevent QuickTime player from opening your files. A problem in your operating system may also cause QuickTime to stop working.
