Join a table with itself
On large tables you may be able to speed it up.

SELECT user_id, user_name 
FROM adquisicion_dispositivo AS a1
WHERE acquisition IS NULL
AND NOT EXISTS (
  SELECT 1
  FROM adquisicion_dispositivo AS a2
  WHERE acquisition IS NOT NULL
  AND a1.user_id = a2.user_id
);


You need to show your table with \d adquisicion_dispositivo. You may need an index, @joanolo's suggestion is good switch the order though I would switch the order,

CREATE INDEX ON adquisicion_dispositivo (user_id, acquisition);