Look up the Mathematica N function in the documentation, change your variable N to n.

But even without that change, if I scrape-n-paste your code into a fresh start of Mathematica it still solves your problem instantly.

So it is possible, but very difficult to prove, that you have previously assigned zero to one of your variables and do not realize this.

Restart Mathematica, try exactly the same problem again and see if the problem disappears.

It is unfortunate that the error message doesn't tell you the name of the variable and that it has been assigned the value of zero. If you saw that you would know exactly what the problem was and how to fix it.