The Excel Num error (#NUM error) will occur when in Excel when it's just not possible to give an answer to a formula based on the inputs of an array of cells.
There is also a possibility of getting the #NUM error from using the wrong arguments in the function, in other words, the function being used is missing or has too many variables in it.
To fix this error take a look at what you are asking of the formula in relation to the cells or array of cells, and also look at how you have written that formula.