What is output of info_lib/0? Because if you have compiled with OpenSSL 1.0 then it will not have Edward’s curves as these are implemented in OpenSSL 1.1 and later. In my case:

iex> :crypto.info_lib()
[{"OpenSSL", 269488207, "OpenSSL 1.1.1d  10 Sep 2019"}]
iex> :ed25519 in :crypto.ec_curves()
true

So there is support for such.

EDIT:

Also if you check edwards_curve_dh/0 type then you will see that you need to use :x25519 instead of :ed25519 as a curve name.

Yes, now this fails, because when signing you need to use :ed25519 (don’t ask me why this works this way). You need to check documentation of crypto:sign/4.

I haven’t used libsalty, but if NaCL is what you need, or you just want to get crypto right without all that hassle, then it is good choice.

In :crypto.generate_key(:ecdh, :ed25519) ecdh refers to all kinds of eliptic curves and then we use x25519. While in :crypto.sign(:eddsa, :sha256, msg, [priv_key, :ed25519]) we have special function for Edwards curves, in which we use ed25519.
I hope it makes a bit more sense.