Query results that not are in previous query result
You are missing the single quote closure after motherboard in both of your queries

AND value != 'Motherboard AND

AND a.value != 'Motherboard

The first query can be more organized as

SELECT id_product 
FROM Atr_basic 
WHERE attribute = 'Product Type' 
AND category = 140
AND value NOT IN ( 'Motherboard', 'Intel Motherboard')