This is NOT a ticket but yet a help for those folks that use Plesk.
If you need to install ffmpeg do it with composer and follow these instructions!
If you need help let me know and I will try and help you.
ffmpeg with composer

For Centos 7
Do the following
whereis ffmpeg
you should see it here
/usr/bin/ffmpeg /usr/include/ffmpeg /usr/share/ffmpeg /usr/share/man/man1/ffmpeg.1.gz

Use: /usr/bin/ffmpeg for your "Path to Encoder"
Make sure open_basedir has this included.

Thought I would add this for all of you and from a old man
Stay Groovy.