Hello, I would like to ask about "unwinnable position" option played on the tournaments. How does it work? 

To justify my question, I will post the screen from my game here, last opponent's move is signed with yellow colour (A7) and my next move would be K9, which no doubt is a move to win. Somehow i still lost by "unwinnable position", as you can see at the bottom. 

Additionally, more than once i had that situation with unwinnable position, when there was still a lot of free space to build 5 in a row winning set. Could someone with more experience clarify the situation regarding to this rule?

Here is the screen which I was talking about: 

https://tinypic.host/i/obraz-2022-06-23-223332628.NJWY9

Thank you in advance for all the answers, have a good day/evening for everyone who reads it!
~Robert :)