Click the cell that displays the error, click the button that appears 
, and then click Trace Error if it appears.
Review the possible causes and solutions.
Possible causes and solutions
Using an incorrect range operator
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To refer to a contiguous range of cells, use a colon (:) to separate the reference to the first cell in the range from the reference to the last cell in the range. For example, SUM(A1:A10) refers to the range from cell A1 to cell A10 inclusive.
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To refer to two areas that don't intersect, use the union operator, the comma (,). For example, if the formula sums two ranges, make sure a comma separates the two ranges (SUM(A1:A10,C1:C10)).
Change the reference so that it intersects.
When you enter or edit a formula, cell references and the borders around the corresponding cells are color-coded.
Color-coded cell references
If there are no squares at each corner of the color-coded border, then the reference is to a named range.
Change references that are not to a named range
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Double-click the cell that contains the formula you want to change. Microsoft Excel highlights each cell or range of cells with a different color.
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Do one of the following:
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To move a cell or range reference to a different cell or range, drag the color-coded border of the cell or range to the new cell or range.
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To include more or fewer cells in a reference, drag a corner of the border.
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In the formula, select the reference, and type a new one.
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Press ENTER.
Change references that are to a named range
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Do one of the following:
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Select the range of cells that contains formulas in which you want to replace references with names.
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Select a single cell to change the references to names in all formulas on the worksheet.
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On the Insert menu, point to Name, and then click Apply.
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In the Apply names box, click one or more names.