Hi there! I understand that your clips are turning black and you're unable to export your project in Movavi on your Mac running OS X 10.15.7. Let's get to the bottom of this together.

To start, can you tell me if this happens with all clips in your project or just certain ones? Also, have you added any heavy effects, filters, or transitions? Sometimes those can cause display or export issues.

Meanwhile, here are some steps to try that often resolve black clips and export problems:

1. Disable Hardware Acceleration
- Open Movavi Video Editor.
- Go to Preferences (usually under the Movavi menu).
- Click the "Acceleration" tab.
- Disable all acceleration options, such as GPU hardware acceleration, then restart the program.

2. Check Export Settings
- When exporting, ensure you select a common format like MP4 with H.264 codec.
- Avoid unusual frame rates or resolutions. For example, use 1080p at 30fps or 60fps.
- Try exporting a short section of your project as a test.

3. Remove Heavy Effects Temporarily
- If you have many effects, titles, or transitions on clips that turn black, try removing them temporarily and attempt export again.

4. Update or Reinstall
- Make sure you have the latest version of Movavi installed.
- If problems persist, consider reinstalling Movavi to rule out corrupted files.

5. Restart Your Mac
- Sometimes a simple restart clears driver or memory issues affecting video export.

If the clips turn black only in the preview but look fine outside Movavi, this might be a video mode or compatibility issue. Adjust video mode settings to 1080p HD at 30fps in your project settings and try again.

Could you check these and let me know how it goes? If it still doesn't work, I can guide you through deeper troubleshooting or help you contact Movavi support.

Are you okay to try these steps?