To create a custom error message in Excel with data validation, try these steps:

1. Select the cell you want to add a data validation rule to.


2. Go to the Data tab and select Data Validation from the Data Tools group.


3. Choose a data validation option from the Allow dropdown and add any additional settings you want.


4. Check the box next to Show error alert.


5. Choose an error style from the Style dropdown.


6. Set the title and message of the error alert.


7. Click OK.