FUNCTION p.IF does not exist. Check the 'Function Name Parsing and Resolution' section in the Reference Manual
SELECT p.IF(LENGTH(`name_ua`), `name_ua`, `name_ru`) AS name, p.IF(LENGTH(`description_ua`), `description_ua`, `description_ru`) AS description, p.in_stock, p.slug, p.productID, p.Price, pics.thumbnail FROM SC_products AS p LEFT JOIN SC_product_pictures AS pics ON p.default_picture=pics.photoID WHERE p.productID="6407" LIMIT 1