Number is not Valid - Match/VlookUp Function within Modify subprocedure

Hello - trying to create a Modify subprocedure which has a Match or VlookUp function within. (what i would like to happen is that, an inputbox appears, user will enter a reference number, then the other information in the form will be filled out)

But I always get an error - Number not Valid. I tried removing the slashes from the lookup/match value using the Substitute function with no luck.

Any help will be appreciated. THANK YOU SO MUCH!