FUNCTION products.IF does not exist. Check the 'Function Name Parsing and Resolution' section in the Reference Manual
SELECT products.productID, products.IF(LENGTH(`name_en`), `name_en`, `name_ru`) AS name, products.Price, products.slug FROM SC_products products LEFT JOIN SC_categories categories USING(categoryID) WHERE products.enabled = 1 AND products.productID <> 4621 AND (products.categoryID = 691 OR categories.parent = 691) LIMIT 5