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 <> 3836 AND (products.categoryID = 766 OR categories.parent = 766)
LIMIT 5