How to count the amount of values grouped by some column?
You really don't need a view for something this simple. But this is what it would look like

CREATE VIEW desired_view
AS
  SELECT client_id, sum(score)
  FROM source
  GROUP BY client_id;


Output 

 client_id |  sum  
-----------+-------
         6 |  3000
         7 | 11000
(2 rows)