It looks like ByRef needs to know the size of the parameter. A declaration of Dim last_name as string doesn't specify the size of the string so it takes it as an error. Before using Worksheets(data_sheet).Range("C2").Value = ProcessString(last_name) The last_name has to be declared as Dim last_name as string *10 ' size of string is up to you but must be a fix length
No need to change the function. Function doesn't take a fix length declaration.