How to use the below expression in my SQL Query
No idea what the is missing is supposed to mean (is it the same as is null?). If yes, the CASE expression and the ORDER BY can be written like this:

ORDER BY
  CASE 
    WHEN Sec.ea_end_of_activity IS NOT NULL
      THEN 2
    WHEN First.aco_ac_ea_id IS NULL 
      THEN 1
    ELSE 0 
  END,
  Sec.aco_acc_short_name ;


The First. and Sec. can be omitted (at least in other DBMS, not 100% sure about Sybase), since they are in the SELECT list.