Is your PCON always a negative value for each period (cash out?). NPV function needs at least one negative value (cash out) and at least one positive value (cash in).
Your NPV will correct itself once you enter the correct cash flows by period.
I think by reading your use case you're expecting .04/12 cash in each month or .646 after your initial investment of -193.8. To get that result you'll need to set up your cash flows like this. We get a number much closer to what you were expecting.
If you have a more complex time series, you can use the 3rd and 4th variable in the function. You can read about it in this article.