To show those that do not exist use this:
R = product.name
From: product, order
Where: product.cod_prod - order.cod_prod
To show those that do not exist use this:
R = product.name
From: product, order
Where: product.cod_prod - order.cod_prod
I believe that this query solves what you need:
SELECT PR.nombre
FROM Producto AS PR
WHERE NOT EXISTS(
SELECT 1
FROM PEDIDO AS PE
WHERE PE.cod_prod = PR.cod_prod
AND YEAR(PE.fecha_ped) = 2018)
I hope it serves you.
Good luck!