You need to do some investigative work as I can't duplicate your error.

You need to confirm the error is an error? What you are using as your lookup value and does it exist in the lookup table?

If you see what you think is a match, test it with comparing the cells. It's as simple as =A1=A1

If they match you will get TRUE and if they don't match FALSE

I understand you said it's another workbook, so just point to those cells which you think match.