Try to update plug_crypto library. It uses :crypto.hmac/3 which was removed in OTP 24.
As you can see on GitHub, newer version of plug_crypto supports OTP 24 plug_crypto/lib/plug/crypto/key_generator.ex at 6ba8fd42a68351459fe6c769ca04aea6022a5cd1 · elixir-plug/plug_crypto · GitHub
