ORA-00904 invalid identifer error when using alias
Aliases cannot be used directly. You have to abstract them out using a nested subquery or derived table.

with t as (
    select
    (select SUM(pest.AREA_HA) 
       from WHSE_FOREST_VEGETATION.PEST_INFESTATION_POLY pest
      where ORG_UNIT_NO IN (1904, 1830,1831, 1902)) 
         as Ha_IN_RKB, 
    (select SUM(pest.AREA_HA) 
       from WHSE_FOREST_VEGETATION.PEST_INFESTATION_POLY pest) 
         as Ha_total
    from dual)
select t.ha_in_rkb, t.ha_total, ROUND(t.Ha_IN_RKB/t.Ha_total,3) from t;