mix deps.get failed (seems missing ssl?)

As @biletskyy answered, it was missing of SSL on Erlang installed via kerl, unlike via brew.

If you want to build Erlang with ssl via kerl, you should pass KERL_CONFIGURE_OPTIONS like below

KERL_CONFIGURE_OPTIONS="--with-ssl=/usr/local/opt/openssl" kerl build 18.0 18.0-foo

Then install Elixir via asdf. The mix command would work well.

Thanks to @biletskyy

In case you arent using kerl and you already did brew install openssl, they added ERLANG_OPENSSL_PATH option to asdf so you can do

asdf plugin-update erlang
ERLANG_OPENSSL_PATH="/usr/local/opt/openssl" asdf install erlang 21.1