Why did TT calculate owing $300 in penalties/interest on this years return when I sold investments in 2016 I had the option of paying taxes on them then or on my return?

guess I am confused why I am paying this penalty and interest. I am paying on this years return the taxes for the investments I sold in 2016. 2016 was the first year ever having sold the total that was way unusual compared to previous years. I don't understand the penalty for doing so. thanks.