When you specify data validation for a cell or range of cells, you can define an error message should the user attempt to enter data that doesn’t match the data validation criteria.
Error Message is optional and is set using the Data Validation dialog - select the Validation command from the Data menu and select Error Alert to see this dialog.
To turn on the Error Alert, check the Show error alert box.
Set the Alert style: choose from Stop, Warning, or Information to display different icons when the user causes the error message to be displayed.
Set a title for the Error Message note for the message in the Title box.
Use the Error Message box to enter the text of the message.
See also:
Defining an Input Message to aid data entry