Excel message: There's a problem with this formula

Well, one possibility is that, just as you've done with posting your question here, you've given the normally very cooperative VLOOKUP function utterly insufficient information to proceed.

That's my smart alec way of saying, "You need to give us a lot more information if you expect a meaningful answer." For example, and just for starters, what's the VLOOKUP formula in this case? Second: is the error message, "There's a problem with this formula"? or is that your interpretation of the situation?

Do you see what I mean? 

In the ideal world--although many of us have had problems doing this--you could help even more by attaching a copy of the workbook(s) in question. That can be a challenge, of course, if they are--or contain materials that are--confidential or proprietary. The point, though, is that you will need to give more information if you want meaningful help.