If you are getting this error when downloading a whole youtube playlist
youtube-dl ERROR: Unable to extract title
Then you need to install the latest version of youtube-dl.
To install the latest version of youtube-dl you can use the PPA webUpd8: ppa:nilarimogard/webupd8 which contains the latest version of youtube-dl.
So first if you have a version of youtube-dl already installed you need to remove it.just type the following line in your terminal.
sudo apt-get remove youtube-dl
Next you need to add the nilarimogard/webupd8 ppa with
sudo apt-add-repository ppa:nilarimogard/webupd8
Next you need to update apt-get cache
sudo apt-get update
Last you just install youtube-dl normally
sudo apt-get install youtube-dl
2018.09.26 is the current version on GitHub
2018.04.03 is the last available on nilarimogard/webupd8 ppa