You must first install Jsonnet.
See the instructions on its GitHub page:
There is also an implementation in Go if you'd prefer:
Simplest way to install Grafonnet is to clone the repository:
git clone https://github.com/grafana/grafonnet-lib.git
A slightly more advanced approach is to use jsonnet-bundler.
jb init jb install https://github.com/grafana/grafonnet-lib/grafonnet
See the Usage page for next steps.