FileNotFoundError: [Errno 2] No such file or directory: 'ffmpeg'

Like the warning says:

Couldn't find ffplay or avplay - defaulting to ffplay, but may not work

You need to have either ffplay or avplay; however ffplay refers to ffmpeg which is not installable in Ubuntu in recent versions. Install the libav-tools package with apt-get:

sudo apt-get install libav-tools

Seems like you need ffmpeg, but

sudo apt-get install ffmpeg 

does not work anymore. You can get ffmpeg by:

sudo add-apt-repository ppa:jon-severinsson/ffmpeg
sudo apt-get update
sudo apt-get install ffmpeg

sudo apt-get install ffmpeg

Note: Tested on Ubuntu 18.04