When building a SQL statement with Postgres and using the UiPath Database Activities package you need to construct the statement manually to provide the encompassing single quotation marks that Postgres expects for a value as the parameter binding method does not work.
Postgres needs $1, $2... for parameters.
UiPath does not use this style by default, so you must write queries using $1, $2 and then add parameters in the same order inside the activity.
INSERT INTO mytable (username, age)
VALUES ($1, $2);
Add parameters in this exact order $1 “John” $2 25
Another example 
query = “INSERT INTO mytable (username, age) VALUES ('” + username + "', " + age.ToString + “);”
How Parameterized Queries and Bind Parameters Are Implemented in PostgreSQL for Efficient Query Execution