Invalid column name 'KodeDiag'

May I ask whats the problem with my SQL Server syntax, I'm using SQL Server 2008.

This my code :

SELECT 
    CASE 
        WHEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Awal IS NULL 
            THEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Akhir
        WHEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Akhir IS NULL 
            THEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Awal
        WHEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Awal IS NOT NULL 
             AND tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Akhir IS NOT NULL 
            THEN tbDiagnosaPasienRekamMedik.Kode_Diagnosa_Akhir
        ELSE '-'
    END AS KodeDiag, 
    dbo.tbDiagnosaICD10.Diagnosa AS NAMA,
    COUNT(KodeDiag) AS JUMLAH
FROM
    dbo.tbDiagnosaICD10 
RIGHT OUTER JOIN
    dbo.tbDiagnosaPasienRekamMedik ON dbo.tbDiagnosaICD10.Kode_Diagnosa = KodeDiag 
WHERE 
    dbo.tbDiagnosaPasienRekamMedik.Tgl_Masuk BETWEEN '2022-06-01' AND '2022-08-31' 
GROUP BY 
    dbo.tbDiagnosaICD10.Diagnosa 
ORDER BY 
    JUMLAH DESC

I get this error from my dbclient:

Msg 207, Level 16, State 1, Server SIMRS-BARU, Procedure, Line 0
Invalid column name 'KodeDiag'.

Msg 207, Level 16, State 1, Server SIMRS-BARU, Procedure, Line 0
Invalid column name 'KodeDiag'

[42S22] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Invalid column name 'KodeDiag'. (207)
[42S22] [Microsoft][ODBC Driver 17 for SQL Server][SQL Server]Invalid column name 'KodeDiag'. (207)

enter image description here

I hope someone can explain what my mistake and how to solve it.

Thank you