Install docbook-xsl-saxon by entering the following commands in the terminal:
sudo apt update sudo apt install docbook-xsl-saxon
Java extensions for use with DocBook XML stylesheets (Saxon)
These are Java extensions for use with the DocBook XML stylesheets and the Saxon XSLT engine. You need these extensions to enable e.g. the callouts and linenumbering extensions of the docbook-xsl suite. . In the past these extensions were shipped with the docbook-xsl tarball distribution directly.