In order for mllint to be able to run the recommended code quality linters, they must be installed in the current environment, i.e. they must be on PATH.
This can be done in a variety of ways, such as installing them globally and / or appending to PATH,
but a more recommended way is to install them into a virtualenv, then activating this virtual environment and running mllint within it.
Poetry and Pipenv do this automatically, simply install them as development dependencies (--dev) and run e.g. poetry shell to open a shell in which to run mllint.