It may happen because someone else accepts them right before you do?
Why does this thing show up often when I select seeks from lobby?

Maybe, but at times, whenever I accept any casual (unrated) challenge from the lobby, this message appears.

It's likely the seek was already accepted by the time you clicked on it
But that happens often in an alt when it clicks casual games (almost everytime it clicks one) . I wonder why it happens. Ig the algorithm doesn't want to match the so-called good sports against the bad sports? Perhaps that might be the reason as well.
When I go to the Custom button under the green "Play" button, it shows the options to edit the timecontrol we desire. After that I click the Open Challenges tab. Now I select a challenge, but often this message shows up, the seek disappears and no game appears.
What does this message mean? Why can't I accept a challenge? Am I not allowed the freedom of accepting any challenge? Why does this even occur, and how do u prevent that? I've seen ppl get exactly what they want in YouTube videos, without these stupid messages.