Unable to install ElPy: "eval-buffer: Cannot open load file: No such file or directory, s"

I followed the instructions to install ElPy from source: I cloned the repo to ~/code/elpy and added these to the Emacs initialization file:

(add-to-list 'load-path "~/code/elpy")
(load "elpy")
(load "elpy-rpc")
(load "elpy-shell")
(load "elpy-profile")
(load "elpy-refactor")
(load "elpy-django")

And I get this error in *Messages*:

Loading ~/code/elpy/elpy.el (source)...
eval-buffer: Cannot open load file: No such file or directory, s

with no more details.

How can I install ElPy on Emacs 27.1 (9.0) and macOS 10.14.6?