Solved my own problem. I forgot to remove ORDER BY clause from each SQL statement and move it to the end. The UNION works when the ORDER BY is at the end.
SELECT "PER"."SSN", RPAD("MED"."PLAN_TYPE",6,' ') AS TYPE, RPAD("MED"."BENEFIT_PLAN",6,' ') AS PLAN, RPAD("MED"."COVRG_CD",2,'...