Just to double check, are you wanting to convert nemo bucks to cookies? It's not possible to convert cookies into nemo bucks.
If you're wanting to convert nemo bucks to cookies, it doesn't seem like you have 4 nemo bucks to convert right now, which is why it's suggesting you top up. If you have 2 nemo bucks, you can click the -15 to get 15 cookies instead, spending only 2 nemo bucks.