How to fix sql create function error "Only one expression can be specified in the select list when the subquery is not introduced with EXISTS."?

Your function is declared with RETURNS INT. That means, that you have to select exactly one single value (=scalar value) to return. In your case your SELECT specifies multiple return columns (a.AFDL, Libur and Aktif).

One thing you should know: Scalar functions (called UDF) are very bad performing. Much better in this case was an inline table valued function (called iTVF). Try it like this (not knowing what you are really doing here...):

CREATE FUNCTION [dbo].[fJatah]
                (@kodewil CHAR (30), @tgl CHAR (19))
                RETURNS TABLE
                AS
                RETURN
                    SELECT a.AFDL, sum(Libur) as Libur, sum(Aktif) as Aktif
                    FROM (SELECT TOP (100) PERCENT a.AFDL, a.kd_wil, ISNULL(MAX(b.libur), 0) AS Libur, ISNULL(MAX(b.aktif), 0) AS Aktif
                    FROM dbo.vM_KEL AS a LEFT OUTER JOIN
                                                                dbo.TblJatahSPTA AS b ON a.KD_KT = b.kd_kt LEFT OUTER JOIN
                                                                dbo.tblSPA AS d ON a.KD_KT = LEFT(d.Kdptn, 5) AND d.tglberlaku = @tgl
                    WHERE a.AFDL = @kodewil
                    GROUP BY a.AFDL,a.KD_KT, a.kd_wil
                    ORDER BY a.AFDL) as a
                    GROUP BY a.AFDL;
GO

SELECT * FROM [dbo].[fJatah]('SomeKodewil', 'SomeTgl')
GO

You can bind this iTVF into your commands with CROSS APPLY or use it where ever you might use a table (FROM, JOIN ...)