Can't execute query

SELECT p.id FROM products_tw p, categories_tw c, products_category_tw as it WHERE p.id = it.pid AND c.id = it.category AND it.category in () AND p.available = '1' AND c.available = '1' GROUP BY p.id

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 ') AND p.available = '1' AND c.available = '1' GROUP BY p.id' at line 1

This script cannot continue, terminating.