If you aim to use Giswater for the first time and need to generate a new work schema, be sure that 'sql' folder isn't empty. There are the files used to generate a new database schema, with empty or sample data. If you downloaded this plugin using Giswater repository 'https://www.giswater.org/descarga' everything is already correct. But downloading directly from Github deserves more attention. You will have to download also 'giswater_dbmodel' repository and copy all the files and folders found there to giswater_qgis_plugin 'sql' folder. Be sure the downloaded branches are exactly the same.