psql export return No such file or directory when the file exist
Change that copy to \copy (https://www.postgresql.org/docs/current/static/app-psql.html#APP-PSQL-META-COMMANDS-COPY). Slightly different command that lets you copy to/from a file to your server.
The reason for the difference here is because you are connecting to a remote server (I assume), but importing a local file.
Another options is to pass your file in and have psql read from stdin like
-c "copy externals (id,logo_path,favicon_path,cover_path,header,description,sign_enable,sign_text,footer_logo_enable,footer_logo_path,footer_text,created_at,updated_at) from STDIN with delimiter as ','" < /Applications/MAMP/htdocs/code/benu/ssc-portal/public/csv/externals.csv