This repository includes submodules, so be sure to do a recursive clone:
% git clone --recursive https://github.com/nd-nuclear-theory/shell.git
You will also need a suitable config.mk file. Most likely you will want to clone the ndconfig repository, if you do not already have it:
% git clone https://github.com/nd-nuclear-theory/ndconfig.git
This is where you will also find the full installation instructions
("Installation instructions for ND nuclear theory projects using ndconfig") in
ndconfig/INSTALL.md.