ORA-00904: "": invalid identifier in Powerbuilder

I am getting an error in datawindow syntax in Powerbuilder. Database Oracle 12c.

Here is the error-

ORA-00904: "SEAT_AVLB": invalid identifier

Here is the code -

 SELECT 0 as sr,
        "TBL_CLASS_MST"."CLASS_NAME",   
         "TBL_CLASS_MST"."SECTION",   
         "TBL_CLASS_MST"."ROOM_NO",   
         "TBL_ROOMS_MST"."MAX_CAP" ,
        TBL_ROOMS_MST.max_cap - (select count(*) from tbl_student_mst) as seat_avlb
    FROM "TBL_CLASS_MST",   
         "TBL_ROOMS_MST"  
   WHERE ( "TBL_CLASS_MST"."ROOM_NO" = "TBL_ROOMS_MST"."ROOM_NO" ) and
        ((class_nr= :ra_class) and
        (seat_avlb>0))

You can't use alias that way; put the whole expression (that makes seat_avlb) into the where clause:

SELECT 0  AS sr,
       "TBL_CLASS_MST"."CLASS_NAME",
       "TBL_CLASS_MST"."SECTION",
       "TBL_CLASS_MST"."ROOM_NO",
       "TBL_ROOMS_MST"."MAX_CAP",
       tbl_rooms_mst.max_cap - (SELECT COUNT (*) FROM tbl_student_mst) AS seat_avlb
  FROM "TBL_CLASS_MST", "TBL_ROOMS_MST"
 WHERE     ("TBL_CLASS_MST"."ROOM_NO" = "TBL_ROOMS_MST"."ROOM_NO")
       AND (    (class_nr = :ra_class)
            AND (  tbl_rooms_mst.max_cap
                 - (SELECT COUNT (*) FROM tbl_student_mst) >
                 0));