My column headings are labeled with numbers instead of letters in Excel for Mac

Cause: The default cell reference style (A1), which refers to columns as letters and refers to rows as numbers, was changed.

Solution: Clear the R1C1 reference style selection in Excel preferences.

Difference between A1 and R1C1 reference styles

1. On the Excel menu, select Preferences.
2. Under Formulas and Lists, select Calculation .
3. Clear the Use R1C1 reference style check box.
   The column headings now show A, B, and C, instead of 1, 2, 3, and so on.