Create an error message

Hi,

I am trying to create a pop up error message using data validation, however as both cells I am using contain formulas it will not work. I have come accross a few posts that have had a similar problem and the suggestions have been to use VBA. I have never used this function in excel or macros before. Can anyone help me here.

Ultimately what I would really like to happen is, I have a reference table on the reference page, which shows a rate code and the age you need to be to be on this rate code.What I want to happen on the Darwin rosters page is that if the rate code (f7) for the employee does not match the age in cell (G7) based on the reference page then create an error.
So the only way I could work out to do this was to create another column that looked up the rate code on the reference page and populated the age, therefore I would then have two columsn to compare and create the error message, but as the Data validation will not work on a formualted cell, I am at a loss.
I have attached the workbook I am using, it is for labour management. So on the Darwin rosters page. What I want to happen is if cell H7 is a lower value than G7, I want an error message to pop up and stop the user from continuing until they change the grade that has been selected.

Can anyone help??