Question

Error: argument of AND must be type boolean, not type character varying

SELECT 
    partno, 
    count(manufacturer) 
FROM 
    components 
WHERE 
    partno IN (SELECT partno FROM productions 
               WHERE 
                    year = 2005
                AND attr is NULL
              ) 
GROUP BY partno
UNION
SELECT 
    partno, 
    count(manufacturer) 
FROM components 
WHERE 
    partno IN (SELECT partno FROM productions 
               WHERE 
                   year = 2005
               AND attr is NULL
              ) 
GROUP BY partno
) 
AND (
        partno NOT IN (SELECT partno FROM components
    )
); 

The part after the union is to include all partno from components that are not in productions (they should be counted as 0)

Answer 1

You have one bracket too many (after the attr IS NULL) and you have an aggregate function (the count in the second part) without a group by. Do you mean this:

select partno, count(manufacturer) 
from components 
where partno in 
(
   select partno from productions where year=2005 and attr is NULL
) 
group by partno

UNION

select partno, count(manufacturer) 
from components 
where partno in 
(
    select partno from productions where year=2005 and attr is NULL
) 
AND partno not in (select partno from components)
group by partno;