pg_stat_statements not found even with "shared_preload_libraries = 'pg_stat_statements'
Recent versions of Postgres install conf files in different locations than they used to be in.  There is sometimes a template .conf file that exists (but is not actually used), and then also an active .conf file (in a completely different location).  I discovered this by scouring through the /etc/init.d files.  It's possible that the .conf file you edited is not actually the one that is being used by the database.

You can find the location of the conf file used by your database by connecting to the database and querying SHOW config_file

Here is a link to your follow up question (in case anyone in the future finds it useful)