Can't execute query

SELECT p.*,c.id as cid FROM products_tw as p, categories_tw c, products_category_tw as it WHERE (p.id = 'oil-kuchaoil' || p.serial_no = 'oil-kuchaoil') AND p.id = it.pid AND it.category=c.id AND p.available = '1' AND c.available = '1' AND it.category in () GROUP by it.pid

MySQL Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') GROUP by it.pid' at line 1

This script cannot continue, terminating.