SQL - HINT to reference a column

I am very beginner at SQL, so I am sorry if this question is primitive.
I just started following the tutorial of http://www.w3school.com, so downloaded the "Northwind" database to try working on it, and using the pgAdmin 3 console to access the DB. I just tried simple command to choose one column of the table, but it gives the same following message with any column from any table

LINE 1: select City from Customers;
                        ^
HINT:  Perhaps you meant to reference the column "customers.City".

I would like to ask is there any thing wrong in my command? and how to fix it?

Thanks