How to fix the ffmpeg not found error with youtube-dl

I love fan noises while sleeping. My sleep quality totally changed for the better since I started.

I was downloading a fan noise audio track from YouTube to put in the Music app using youtube-dl, like this:

youtube-dl ciD52cwJGCs --extract-audio --audio-format mp3 --prefer-ffmpeg; 

But after downloading the file, I got the error

ERROR: ffprobe/avprobe and ffmpeg/avconv not found. Please install one.

To solve this on my Mac I had to run

brew install ffmpeg

It took a while, then I ran the command again and since the file was already downloaded, it was a quick process to get the mp3.