Install libocp-indent-lib-ocaml-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libocp-indent-lib-ocaml-dev
OCaml indentation tool for emacs and vim - development libraries
ocp-indent is a command-line tool that allows one to indent a whole OCaml source code file (or parts of it) either to standard output or in-place. A configuration file allows user defaults as well as per-project parameters. The ratio of correctly indented lines is comparable with emacs tuareg mode while being an order of magnitude faster. . This package includes the development libraries.