You cannot perform a sum operation in Excel

When Excel edits the data, you cannot perform a sum operation.

This is because when you edit the data, you change the value of the field, causing the data in all the cells in the sum field to be unchanged, resulting in a situation that cannot be calculated.

You can select "Tools" → "Options", in the Options dialog box that opens, click the Recalculation tab, select Automatic recalculation in the calculation area, click OK, and when you are finished, you can automatically calculate and update the values of the cells.