It's a known issue and should be fixed in the next release. You can safely ignore it by clicking the button on the bottom left.
Viewing post in Tales From The Unending Void v0.21 comments
Hello ! Have you figured out the issue with the mac version not running at all due to missing "gui" (as reported below) ? SL was an amazing game and TFTU is even better, i was eager for episode 21. Do you think you'll be able to address the issue in the near future or will mac users have to wait for episode 22 to play episode 21 ?