如何计算excel中的非相邻空白单元格？

I am working on a search function that searches a table in another worksheet that contains client information, from a cell using VLOOKUP. I have 5 "search box" cells and want to be able to search by one criteria at a time, but only if the other 4 search Criteria boxes are empty. So if there was text in more than one of the search boxes at once it would return an error message. I am working on a search function that searches