Implement SQL query with conditional sum and how does it work?

I am trying to write the SQL to generate the count of unique users who have purchased product B but have never purchased product C.

Someone suggested trying the following

SELECT COUNT(*) AS bought_b_no_c
FROM (
  SELECT user_id
  FROM table_purchases
  WHERE product IN ('B', 'C')
  GROUP BY user_id
  HAVING SUM(product = 'C') = 0
) t;

However, SQL Server is saying I have an error as follows:

Msg 102, Level 15, State 1, Line 5 Incorrect syntax near ‘)’.

Anybody know why this is happening and how to learn exactly how this work?

PS This was my original code

SELECT Count(*) AS bought_b_no_c
FROM   (SELECT user_id,
               Sum(bought_b_no_c) AS boolean_b_no_c
        FROM   (SELECT user_id,
                       product,
                       CASE
                         WHEN product = 'B' THEN 1
                         WHEN product = 'C' THEN -1
                         ELSE 0
                       END AS bought_b_no_c
                FROM   table_purchases) AS T
        GROUP  BY user_id) AS J
WHERE  boolean_b_no_c = 1