PostGIS ST within does not exist

Your problem is that you have excluded the public schema from the search_path. All functions and types is in the public schema (by default)

So what you have to do is just include the public schema in the search_path

SET search_path TO sml, public;

means that the database will write to sml if you don't give any schema

SET search_path TO public, sml;

means that the database will write to public if no schema is given.

But in both cases both schemas will be searched for tables, functions and so on