Really appreciate all the help here. Looking at your post:

1. deps/argon2_elixir does contain a folder argon2. Within that folder there is an include folder that has a single file, argon2.h, and a src folder which has 14 files most of which are .c files, with some .h files.

Trying number 2 now, and will update with the results

EDIT: Thinking your git comment was right, as this is in the output when I tried recursively cloning argon2_elixir:

git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
fatal: clone of ‘git@github.com:riverrun/phc-winner-argon2.git’ into submodule path ‘/Users/steve/Sites/argon2_elixir/argon2’ failed
Failed to clone ‘argon2’. Retry scheduled

Ok so after adding an SSH key I was able to clone the repo properly. From within that directory I ran make and got

steves-mbp:argon2_elixir steve$ make
mkdir -p priv
cc -pthread -O3 -Wall -g -Iargon2/include -Iargon2/src -Ic_src -I"/usr/local/Cellar/erlang/22.2.8/lib/erlang/erts-10.6.4/include/" -dynamiclib -undefined dynamic_lookup argon2/src/argon2.c argon2/src/core.c argon2/src/blake2/blake2b.c argon2/src/thread.c argon2/src/encoding.c argon2/src/ref.c c_src/argon2_nif.c -o priv/argon2_nif.so
steves-mbp:argon2_elixir Steve$

Since this didn’t complain about any errors, I took it that mix was working. So I went back to my project, and completely removed argon2_elixir and comeonin. I then re-added argon2_elixir and ran mix deps.get and mix deps.compile and got the following output:

steves-mbp:rumbl steve$ mix deps.compile
===> Compiling ranch
===> Compiling telemetry
==> comeonin
Compiling 4 files (.ex)
Generated comeonin app
==> argon2_elixir
mkdir -p priv
cc -pthread -O3 -Wall -g -Iargon2/include -Iargon2/src -Ic_src -I"/usr/local/Cellar/erlang/22.2.8/lib/erlang/erts-10.6.4/include/" -dynamiclib -undefined dynamic_lookup argon2/src/argon2.c argon2/src/core.c argon2/src/blake2/blake2b.c argon2/src/thread.c argon2/src/encoding.c argon2/src/ref.c c_src/argon2_nif.c -o priv/argon2_nif.so
Compiling 3 files (.ex)
Generated argon2_elixir app
===> Compiling cowlib
===> Compiling cowboy

Everything seemed to work fine, but I’m still seeing the error, could not compile comeonin

No, not really, it is the recommended password hashing function for comeonin, and depends on it.

It would be nice to know how You did that… with asdf? packages? other?

Do You have XCode installed?

That sounds like a Mac related issue

Most people prefer to use a package manager…

I also prefer to use asdf-vm.

You probably can find some topics to help You if You choose this installation path.