Below working code. Just make sure that when you passing variables to string barCode -- it is not empty or nulls. Single null in that string formation will ruin your day.
string barCode = barCodeTrans + barCodeSeq + barCodeIndex + rotationindex.ToString();
Lesnikowski.Barcode.Barcode128 bc = new Lesnikowski.Barcode.Barcode128();
bc.Rotation = Lesnikowski.Barcode.RotationType.Degrees90
bc.Number = barCode;
bc.CustomText = "";
bc.Height = 28;
bc.NarrowBarWidth = 2;
Bitmap bcBitMap = bc.GenerateBitmap();
string fileName = barCode + ".jpg";
bcBitMap.Save(outputDir + "/" + fileName, ImageFormat.Jpeg);
return fileName;