With reference to this, I tried as follows. Use -h to get full help or, even better, run 'man ffmpeg'

Ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developersīuilt with gcc 9 (Ubuntu 9.3.0-10ubuntu2)Ĭonfiguration: -prefix=/usr -extra-version=1ubuntu0.1 -toolchain=hardened -libdir=/usr/lib/x86_64-linux-gnu -incdir=/usr/include/x86_64-linux-gnu -arch=amd64 -enable-gpl -disable-stripping -enable-avresample -disable-filter=resample -enable-avisynth -enable-gnutls -enable-ladspa -enable-libaom -enable-libass -enable-libbluray -enable-libbs2b -enable-libcaca -enable-libcdio -enable-libcodec2 -enable-libflite -enable-libfontconfig -enable-libfreetype -enable-libfribidi -enable-libgme -enable-libgsm -enable-libjack -enable-libmp3lame -enable-libmysofa -enable-libopenjpeg -enable-libopenmpt -enable-libopus -enable-libpulse -enable-librsvg -enable-librubberband -enable-libshine -enable-libsnappy -enable-libsoxr -enable-libspeex -enable-libssh -enable-libtheora -enable-libtwolame -enable-libvidstab -enable-libvorbis -enable-libvpx -enable-libwavpack -enable-libwebp -enable-libx265 -enable-libxml2 -enable-libxvid -enable-libzmq -enable-libzvbi -enable-lv2 -enable-omx -enable-openal -enable-opencl -enable-opengl -enable-sdl2 -enable-libdc1394 -enable-libdrm -enable-libiec61883 -enable-nvenc -enable-chromaprint -enable-frei0r -enable-libx264 -enable-shared

Ffmpeg build enablelibx264 not found