Postgres error with Power BI Desktop

I had the same issue and it turned out to be easy to solve if you can get the right information. As a Windows application PowerBI uses the windows certificate store to decide what to trust. If you can get the SSL cert for the PostgreSQL server (or the CA cert used to sign that one) then tell Windows to trust that certificate, PowerBI will trust it too.

To get the SSL cert for the PostgreSQL server you might need to speak to the DBA/sysadmin if you don’t have access. In the configuration folder for the PostgreSQL server there is a postgresql.conf file, search it for ssl settings, there is one with the location of the ssl cert. Note NOT the key file which contains the private key, only the cert file which contains the public key. copy it or its content to the machine running PowerBI and import using Run | mmc | Add Plugin... Certificates (Google it)

Look at the server name once you imported the cert and connect from PowerBI using the same server name (so the cert matches the connection). That solved the problem for me. If PostgreSQL is configured to insist on a SSL connection you might have to do the same for a ODBC connection too.