Try setting the environment variable HOUDINI_SCRIPT_LICENSE to hbatch -R in your houdini.env file.

This will attempt to check out your non-graphical license (Engine) rather than consuming your interactive FX or Core licenses.

Running hserver -n does not work with hython or with python based tools inside a file.

For more information about Houdini environment variables: www.sidefx.com/docs/houdini/basics/config_env.html