I encountered the same error when upgrading and old project from OTP22/Elixir 1.9.4 to OTP24/Elixir 1.14.
If I read the documentation correctly the solution is to now use :crypto.mac functions, as the :crypto.hmac/3 is no longer present in OTP24.
In my case I replaced my :crypto.hmac/3 call with :crypto.mac/3
