This is because cells are formatted as text and can be formatted as normal. The specific solutions are as follows:

1. First, select a cell. You also need to select a cell to sum, and then drop down the box at the arrow to format the cell.

2. Then in the pop-up window, set the format to "General", as shown in the figure below, and then go to the next step.

3. Next, enter the following summation formula, and then proceed to the next step.

4. Finally, press Enter to get the sum data, as shown in the figure below.