MAXX measure wrong value

Hi eunji888888 Try adding this in the filter after Value 'Combined Group Discount'[Material Group.Material] = CurrentMaterial inside calculate here is the full code 

Discount Measure =
VAR CurrentMaterial = SELECTEDVALUE('By SO'[Material])
VAR CurrentNetCase = SUM('By SO'[NetCase])
VAR CurrentBillingDocNo = SELECTEDVALUE('By SO'[BillingDocNo])
VAR FilteredScalarValue =
    CALCULATE(
        MAXX(
            FILTER(
                'Combined Group Discount',
                'Combined Group Discount'[Material Group.Material] = CurrentMaterial
            ),
            VALUE(RIGHT(
                'Combined Group Discount'[Column],
                LEN('Combined Group Discount'[Column]) - FIND("_", 'Combined Group Discount'[Column])
            ))
        ),
        'Combined Group Discount'[Material Group.Material] = CurrentMaterial//Explicily add this
    )
VAR IsScalarValueFound = NOT(ISBLANK(FilteredScalarValue))
RETURN
IF(
    IsScalarValueFound && CurrentNetCase >= FilteredScalarValue,
    "apply", // Replace with your discount logic
    "No Discount"
)