You should have installed OCaml 4.02.3 and ppx_let.
$ pwd
path/to/hooks-exer
$ mkdir ppx_let
$ cd ppx_let
$ yarn init
$ opam switch 4.02.3
$ ln -s `opam config var ppx_let:lib`/ppx ppx
$ yarn link
$ cd ..
$ yarn link ppx_let
$ yarn build
$ yarn webpack
$ yarn server
$ yarn test