You need to have opam >= 2.0 installed.
The recommended way is to run the following commands, which create a local opam switch, install dependencies and compile Coq proofs:
opam switch create . -y --deps-only --repos default,coq-released=https://coq.inria.fr/opam/released
eval $(opam env)
make
Alternatively, you can try to install dependencies in the current opam switch and compile Coq proofs:
make depend
make