When you "guess schema" it doesn't read the DB. It reads the Query, it tries to build a schema from the query in the "Query" box. If there is no query, then you get an error.
This is similar to guessing the Query. It doesn't read the DB, but instead reads the specified Schema in your component
So, when you create a new DB connection in the Metadata/Db connections you can right click and "Retrieve Schema" from your DB. Then you can assign the schema to your DB component, then you can use "Guess Query".
