Not much to go on here but a couple of guesses.
If the formula is an array formula it must be entered using CTRL + SHIFT +
ENTER or you will get the error.
If one or more of the values is not really a number, you will get the error.
If any of the referred to cells has a formula with an error you will get the
error.
Perhaps post a sample(not a file) of your data and formula.
Gord Dibben Excel MVP