Cannot start observer from IEx shell on MacOS?

This means you don’t have wxWidgets installed.

I have Erlang installed with asdf tool, highly recommended using it for Erlang and Elixir installations.
So if you have Erlang already installed, uninstall it first:

Then you need to install wxWidgets:

Check whether you have wxWidgets installed correctly:

(it should show a version number)

After that you need to install Erlang back:

Voila, you can start observer from your IEx shell: