Unknown encoder 'libx264'

I am using Ubuntu 11.04, and I also came across this error - Unknown encoder 'libx264'. Installing the package libavcodec-extra-53 resolved the problem.

In Ubuntu 14.04 LTS the package that is needed is called libavcodec-extra-54 (and libav-tools)

start by installing these libraries

sudo apt-get install libfdk-aac-dev libass-dev libopus-dev  \
libtheora-dev libvorbis-dev libvpx-dev libssl-dev

For Ubuntu 20.04 issue

sudo apt-get install nasm

and hop over this manual nasm install ... yet as of Ubuntu 17.04 sudo apt-get install nasm does not supply a new enough nasm so install that manually

get source for nasm at http://www.nasm.us/pub/nasm/releasebuilds/?C=M;O=D download the latest then

cd ~/src/nasm-2.13.02  #  update to release you just downloaded
./configure
make -j $(nproc)
sudo make install

Then for x264 :

git clone https://code.videolan.org/videolan/x264.git
cd x264
./configure --enable-static --enable-shared
make -j $(nproc)
sudo make install

For mp3 get LAME (libmp3lame)

sudo apt-get install libmp3lame-dev

or install manually from http://lame.sourceforge.net/ version v3.100, then give it the normal

cd lame-3.100/
./configure
make -j $(nproc)
sudo make install

Finally download ffmpeg source and do install :

git clone git://source.ffmpeg.org/ffmpeg.git
cd ffmpeg

./configure  --enable-gpl   --enable-libass   --enable-libfdk-aac   --enable-libfreetype   --enable-libmp3lame   --enable-libopus   --enable-libtheora   --enable-libvorbis   --enable-libvpx   --enable-libx264   --enable-nonfree --enable-shared --enable-openssl   

make -j $(nproc)
sudo make install

I feel your pain, but this works - still valid on Ubuntu 18.10