There are many reasons why a connection can be refused. Is dbHost a localhost?

Try

conn = OpenSQLConnection[
    JDBC["PostgreSQL", "<host_name>/<db_name>"],
    "Username" -> "<user_name>", "Password" -> "<password>"
    ]

E.g to connect to a public RNA sequence PostgreSQL database named pfmegrnargs hosted by RNACentral

conn = OpenSQLConnection[
    JDBC["PostgreSQL", "hh-pgsql-public.ebi.ac.uk/pfmegrnargs"],
    "Username" -> "reader", "Password" -> "NWDMCE5xdipIjRrp"
    ]

tables = SQLTableNames[conn]

Execute query using SQL string (WL style SQL queries would be an alternative)

SQLExecute[conn, "select * from xref where ac = 'OTTHUMT00000106564.1'"]

Close connection

CloseSQLConnection[conn]