Can't install ffmpeg on macOS High Sierra 10.13.6
Solution 1:
Download ffmpeg for macOS from evermeet.cx.
Put it in /usr/local/bin and you can execute it from any directory.
Consider donating if you find it useful. I'm not affiliated with evermeet, but the owner says, "over the past 10 years there were about 12 donations - in total less than $250. I never wanted to make money with those binaries, but I almost can't afford the traffic anymore".