00933 error - Cannot find where I have gone wrong

I am writing this query which joins 3 tables together, I then need to find best performing categories for which I shall use rank. I have written the query below but I get a 00933 on line 3 and cannot work out what I have done incorrectly.

 SELECT S98.PRODUCT_ID, SUM(S98.STORE_SALES) AS SALES, PC.CATEGORY,
  RANK() OVER (ORDER BY SALES DESC)
  FROM SALES_98 AS S98, PRODUCT_CLASS AS PC
INNER JOIN P ON S98.PRODUCT_ID = P.PRODUCT_ID
INNER JOIN PC ON P.PRODUCT_CLASS_ID = PC.PRODUCT_CLASS_ID
UNION ALL
SELECT SD98.PRODUCT_ID, SUM(SD98.STORE_SALES) AS SALES, PC.CATEGORY,
  RANK() OVER (ORDER BY SALES DESC)
  FROM SALES_JAN_98 AS SD98, PRODUCT_CLASS AS PC
INNER JOIN P ON SD98.PRODUCT_ID = P.PRODUCT_ID
INNER JOIN PC ON P.PRODUCT_CLASS_ID = PC.PRODUCT_CLASS_ID