Hi opn6,

I do not believe the problem is the formula that REFERS TO cell B10. I SUSPECT the issue is that B10 itself has data validation, and that this message you are seeing was created by whoever created the data validation for that cell.

To test my theory ...
Select cell B10
from the Excel menu select Data >> Validation ...
This opens the Data Validation dialog.
On the "Settings" tab, what do you see in the Allows: box?
On the "Error Alert" tab, what do you see in the Error Message box?