Use an Alias in Where Clause Subquery
you cant use alias from the wraping query in the a sub query
change it this way 

SELECT
ANGGARAN.SIMPEG_PEGAWAI.ID_PEGAWAI AS KODE,
ANGGARAN.SIMPEG_PEGAWAI.NAMA,
ANGGARAN.SIMPEG_PEGAWAI.NIP,
ANGGARAN.SIMPEG_ESELON_JABATAN.JABATAN,
ANGGARAN.SIMPEG_KODE_GOLONGAN_PANGKAT.GOLONGAN,
ANGGARAN.SIMPEG_KODE_GOLONGAN_PANGKAT.PANGKAT,
    (SELECT *
            FROM (SELECT CONCAT(TO_CHAR(abs(sysdate - TO_DATE(TMT_JABATAN))/360,'9,999,999.9'),' TAHUN') 
                FROM SIMPEG_JABATAN where ID_PEGAWAI=ANGGARAN.SIMPEG_PEGAWAI.ID_PEGAWAI  ORDER BY TMT_JABATAN desc)
    WHERE ROWNUM = 1) AS MASA_KERJA
FROM 
ANGGARAN.SIMPEG_PEGAWAI
    INNER JOIN ANGGARAN.SIMPEG_ESELON_JABATAN 
        ON ANGGARAN.SIMPEG_PEGAWAI.ESELON_JABATAN = ANGGARAN.SIMPEG_ESELON_JABATAN.ID_ESELON_JABATAN
    INNER JOIN ANGGARAN.SIMPEG_KODE_GOLONGAN_PANGKAT 
        ON ANGGARAN.SIMPEG_PEGAWAI.PANGKAT = ANGGARAN.SIMPEG_KODE_GOLONGAN_PANGKAT.ID_GOLONGAN_PANGKAT
WHERE
    ANGGARAN.SIMPEG_PEGAWAI.ST_AKTIF = 1 AND
    ANGGARAN.SIMPEG_PEGAWAI.ESELON2 &lt;&gt; 1 AND
    ANGGARAN.SIMPEG_PEGAWAI.PANGKAT &gt;= 12 AND 
    ANGGARAN.SIMPEG_ESELON_JABATAN.STATUS = 1 AND
    ANGGARAN.SIMPEG_ESELON_JABATAN.ID_ESELON2=2
ORDER BY
ANGGARAN.SIMPEG_KODE_GOLONGAN_PANGKAT.SORT DESC


remember, sql engine process a query in the following order 


from 
where
group by 
having 
select


so, when you execute the inner query the sql engine does no know the aliad KODE yet.