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Cause: The formula uses a custom function that is not available.
Solution: Make sure that the workbook that contains the custom function is open and that the custom function is working correctly.

Select the cell.

On the Formulas tab, under Function, click Formula Builder.

Edit the formula in the Arguments section of the Formula Builder window.
Note: Some builtin functions that are available in Excel for Windows are not available in Excel for Mac. To learn more, see My function doesn’t work.
Cause: The formula contains a reference to a named cell or range that is not defined.
Solution: Make sure that the named cell or range is defined.

On the Insert menu, point to Name, and then click Define.

If the named cell or range is not listed, click Define to add it.
Cause: The formula contains a misspelled function name, named cell, or named range.
Solution: Edit the formula to correct the spelling.

Select the cell.

On the Formulas tab, under Function, click Formula Builder.

Edit the formula in the Arguments section of the Formula Builder window.
Note: Some builtin functions that are available in Excel for Windows are not available in Excel for Mac. To learn more, see My function doesn’t work.
Cause: The formula contains text that is not entered correctly.
Solution: Edit the formula to enclose the text in double quotation marks ( " ).

Select the cell.

On the Formulas tab, under Function, click Formula Builder.

Edit the formula in the Arguments section of the Formula Builder window.
Cause: A colon ( : ) is missing in a range reference.
Solution: Make sure that all range references in the formula use a colon ( : ).
For example, =SUM(A1:C10).
Cause: The formula contains a reference to another workbook or sheet with a nonalphabetic character or space in its name, but the reference is not enclosed in single quotation marks ( ' ).
Solution: Edit the formula to enclose the workbook or sheet name in single quotation marks ( ' ).

Select the cell.

On the Formulas tab, under Function, click Formula Builder.

Edit the formula in the Arguments section of the Formula Builder window.
Cause: The formula contains a reference to a row or column title (labels), which is not supported in Excel 2008 for Mac and Excel 2007 for Windows.
Solution: Allow Excel to change label references to standard cell references.
When you open a workbook that contains formulas with labels, Excel gives you the option to change the label references to standard cell references. For example, if a workbook contains a column of data with the title "Sales" that spans from cell A2 to cell A10, and the workbook contains the formula =SUM(Sales), Excel replaces the label reference "Sales" with the standard cell reference "A2:A10" and then recalculates the formula. If you allow Excel to make these changes, after you save the workbook, the changes to the formula become permanent and cannot be undone.
You can also use names to create formulas that are easier to understand. For more information about how to create names and formulas that use names, see Use names in formulas.