Install debiandoc-sgml by entering the following commands in the terminal:
sudo apt update sudo apt install debiandoc-sgml
DebianDoc SGML DTD and formatting tools
This is an SGML-based documentation formatting package used for the Debian manuals. It reads markup files and produces HTML, LaTeX, DVI (via LaTeX), PostScript (via LaTeX/DVI), PDF (via LaTeX), Texinfo, Info (via Texinfo), DebianDoc SGML, DocBook XML, plain text (with overstrikes a la troff as well as without) files. . The LaTeX based output needs the texlive, texlive-latex-extra and texlive-lang-all packages. In addition to this, Chinese and Japanese need latex-cjk-all and corresponding font packages. . The Texinfo based output needs the texinfo package.