Tested and working for me.
IF ($[PDN1.UnitMsr]) = 'Nos'
BEGIN
Select (ABS($[$38.11.Number])-($[$38.U_GrDcQty.NUMBER]))
END
ELSE
BEGIN
Select (ABS($[$38.11.Number])-($[$38.U_GrDcQtyKG.NUMBER]))
END
Note: .U_GrDcQty - Numeric type
U_GrDcQtyKG Alphanumeric type