It looks like it’s seeing the pre-populated values as string values, not numbers. If you take those out, and enter your own numeric values, it calculates through.
That said, something is going wrong with the calculation - 10 items at 12 (units), should not come out at $1209.90, even allowing for tax. Interestingly, if you add decimal places to the ‘price’ input, that also generates a NaN result.