[abelg]

Hola creo que en updates no puedes definir alias y la cosa quedaría asi:

Código SQL [-]
UPDATE ps_orders
SET 
tienda_order =  
CONCAT( 
        id_customer,
        '-',          
        (SELECT count( d.id_customer ) + 1 
         FROM ps_orders d          
         WHERE d.id_customer = ps_orders.id_customer AND d.id_order < ps_orders.id_order)
) 
WHERE id_order =901