An error with youtube-dl

Make sure that you have the libavcodec-extra-53 package for encoding to mp3 installed. To install libavcodec-extra-53 from the terminal run:

sudo apt-get install libavcodec-extra-53  

Libav codec library:
Libav is a complete, cross-platform solution to decode, encode, record, convert and stream audio and video.

• In Ubuntu 14.04 the Libav codec library (libavcodec-extra-53) has been updated to libavcodec-extra-54.
• In Ubuntu 15.10 and 16.04 the Libav codec library has been updated to libavcodec-ffmpeg-extra56.
• In Ubuntu 16.10, 17.04, 17.10, and 18.04 Libav codec library has been updated to libavcodec-extra57.
• In Ubuntu 18.10, 19.04, 19.10, 20.04, 20.10 and 21.04 Libav codec library has been updated to libavcodec-extra58.