getitem () работает, но getactiveworksheet () не работает

Question

type Getitem("Sheet4"). This snippets works.
But change to *worksheets.getActiveWorksheet()* methods then the whole snippet not work. 

Так в чем разница между getitem и getactiveworksheet.

   await Excel.run(async (context) => {
        let sample = context.workbook.worksheets.getItem("Sheet4");
        let sourcerange = sample.getRange("C1");
        sourcerange.load("format/fill/color, format/font/name, format/font/color");
        return context.sync().then(function() {
          var targetrange = sample.getRange("c6").getSurroundingRegion();
          targetrange.set(sourcerange);
          return context.sync();
        });