How to incorporate validation list in custom validation?

I have an excel file, I want to prompt user that the cell B7 is not available while value of cell B4 is "N/A", cell B7 is a drop down list.

I want to add =ISERROR(SEARCH("N/A",$B$7)) to custom validation in cell B7 but meanwhile I need B7 as a drop down list. How can I make that?

is there any formula like =if(ISERROR(SEARCH("N/A",$B$7)), prompt error, else let user choose from drop down ?

Thanks