Error trying to update stock with trigger
You cannot use the reference PRODUCT_SALES. for columns inside the trigger.

In the code, once you get passed FOR EACH ROW you must reference the new value with NEW.

CREATE TRIGGER updateStock
AFTER INSERT
ON PRODUCT_SALES
FOR EACH ROW
    UPDATE PRODUCTS
    SET PRODUCTS.stock = PRODUCTS.stock - NEW.quantity
    WHERE PRODUCTS.id_product = NEW.product_id;