https://github.com/PJUllrich/www_redirect/blob/main/lib/mix/tasks/www_redirect.install.ex#L118

Somewhere around there (but not in the case that the first thing matched, since that means its already installed), there should be something like:

{:notice,  """
Could not install WwwRedirect. We look for Plug.Static, and add our plug just before it.
However, we could not find that plug in your endpoint. Please add the following line in your endpoint:

plug WwwRedirect, to: :non_www
"""}