Hi Vlada, was this a batch campaign or a triggered campaign? I'm willing to bet it was batch -- where A/B testing won't work.
I've gotten around this by having a batch campaign to add people to a list, and being added to that list activates a triggered campaign for the A/B subject line send.
Another thing I'll do is create two separate emails (without the A/B test), then create a batch campaign that has a send where the flow step uses the choice "Random sample is 50%" and it sends one of the emails, and the default option is the other email. I prefer this option because it keeps the reporting on the two emails forever whereas the reporting shown in your image above will disappear once you choose a champion.
Here's an image to clarify the second option described above: