Why does ffmpeg say "cannot find preset"?

Question

When trying to transcode an input to x265 ffmpeg tells me "preset veryslow not found":

ffmpeg -i <in_file> -vf scale=xxx:yyy -c:v hevc -preset veryslow <out_file>

Answer 1

ffmpeg needs to be compiled with both the corresponding development versions of the required libraries and the corresponding encoders/decoders enabled when invoking configure at the beginning of the build process.

In case of the Debian, this would include libx265-dev and the configure option --enable-libx265 for the corresponding x265 / HEVC preset to be included in the corresponding binary.