Before using ffmpeg-python, FFmpeg must be installed and accessible via the $PATH environment variable.
There are a variety of ways to install FFmpeg, such as the official download links, or using your package manager of choice (e.g. sudo apt install ffmpeg on Debian/Ubuntu, brew install ffmpeg on OS X, etc.).
Regardless of how FFmpeg is installed, you can check if your environment path is set correctly by running the ffmpeg command from the terminal, in which case the version information should appear, as in the following example (truncated for brevity):
$ ffmpeg
ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developers
built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)