You are missing the link between the two tables. You could write it like this:
/* not the best option */
UPDATE TABLE_1
SET N_VALUE =
(SELECT P_VALUE
FROM TABLE_2
WHERE P_FID IN (SELECT N_FID
FROM TABLE_1
WHERE N_VALUE = '21000')
AND p_fid = n_fid)
or this:
UPDATE TABLE_1
SET N_VALUE =
(SELECT P_VALUE
FROM TABLE_2
WHERE 1 = 1
AND p_fid = n_fid)
WHERE N_VALUE = '21000'