Solution 1: Update Software

When QuickTime is not working on Mac, the first thing you should do is to update your software.

Step 1: Click the Apple menu, go to System Preferences and choose Software Update.

Step 2: If there is an updated available, click the Update Now button and follow the onscreen instruction to install it.

Step 3: Wait for the process to finish, reboot your computer, and open your videos in QuickTime again.

Solution 2: Install Codec Package

QuickTime is not working partly because your video formats are not compatible. The multimedia formats compatible by QuickTime are MOV, most MPEG files, some AVI and WAV files. If your video does not play in QuickTime, you have three options:
1. Look for another media player, such as VLC.
2. Convert your video files to QuickTime supported formats, like MOV.
3. Install extra codec for QuickTime. You can search for QuickTime codec page in Google, download and install the proper one. Then you can play more video formats in QuickTime.

How to Fix QuickTime Screen Recording Not Working: https://www.vidmore.com/record-video/quicktime-not-working-on-mac/#part2