The package not installed.

If coal has been installed more recently than this documentation, you need to configure and install again this documentation.

Otherwise, if coal is not installed, you can install using one of the following steps:

get the source code through git:
git clone git+https://github.com:humanoid-path-planner/coal

Then configure, compile and install the package.

Note that you will need to configure and install again this documentation to activate the link to coal.