Now that Spack is installed, you can install HEPnOS and its dependencies:
spack install hepnos
This should install all the dependencies. Note that MPI and Boost are two potentially large dependencies that will take some time to compile. If you already have MPI and/or Boost installed on your system (without Spack), you can tell Spack where to find them so it doesn't try to compile them.
Once installed, HEPnOS and its dependencies can be loaded as follows: