Confusion over paying this years Income Tax.

This is my 2nd year of paying tax self employed. I paid it last year (2017-2018) and It asked for the upfront 1st six months payment for (2018-2019) then 6 months later I pay the 2nd half.
Now that I'm completing my form for 2018-2019. Its given my total but also says 'that the total isn't subtracting any previous payments made for 2018-2019.

My confusion is how do I deduct those payments from the total before I submit my tax return otherwise If I don't im going to be paying that same amount again? Plus now it also asks for the 1st 6 month payment of 2019-2020. I really wish it was clearer or just deducted it itself since it asks for it. I'm not very knowledgeable about this so any help would be greatly appreciated.