You opened index.html directly. The MP4 button needs the local Python/FFmpeg server, so Chrome cannot use it from a file:// page.
Do this:
1. Close this tab.
2. On Mac, double-click START TOOL.command. On Windows, run START TOOL WINDOWS.bat.
3. Keep the Terminal/Command Prompt window open.
4. The correct page will open automatically at http://127.0.0.1:8765.
5. Then Download MP4 will work with one click.
Do not open index.html directly. Always use the launcher for your operating system.