to help you please better describe us your case and the followed steps.
First of all you can verify the string connection to Postgres and test it with another dbms manager tool through the same jdbc driver used (imported) in WebRatio.
Please check the presence of any restriction due to firewall rules or network configuration (protocol, host, port and so on) or Postgres security settings.
Keep in mind that you have to use the correct version of the jdbc driver that must be compatible with the version of Postgres (e.g. 9.4.x) and java (e.g. Oracle jdk 1.7) in use. For more detail please refer to the official Postgres donload page with compatibility table: https://jdbc.postgresql.org/download.html.
Moreover please refer to this guide https://my.webratio.com/learn/learningobject/connecting-the-database-v-72.