Macro for Lookups

[rhmkrmi]

Hi there,

I need a macro with a pop-up message for a lookup.

The formula I am using is =INDEX('My Sheet'!B:B,MATCH("Criteria"&A1,'My Sheet'!A:A&'My Sheet'!B:B,0))

If the formula finds the value, the macro should pop-up a message and stop the user from continuing until a unique value is entered in column A.

I also need to make it persistent so the user can't proceed before the value is fixed, that is to say, the lookup can't find it.

Thank you.

 

[rhmkrmi]

Anyone?