So the output you saw would have been from the VSCode output window (View → Output) - but that’s no use to you!
Having a quick look at the readme for the first neovim plug-in that supports ElixirLS (GitHub - elixir-tools/elixir-tools.nvim: Neovim plugin for Elixir · GitHub), there’s a section explaining how to access the ElixirLS output - search for OutputPanel.
