Sorry for the late response! I'm playing on Windows :)
Alrigh, I (finally) looked at it.
Apparently, the itch app opens HTML5 games in its own browser, which doesn't support the prompt command. I can't fix that!
But if you (or anybody reading this) wants to play the game downloaded via the app, you can do it properly by clicking the option gear, then Manage, and Open folder in explorer.
Then click RoboCreator_by_RoboCorp.html and open it in your preferred browser, which most likely will support the prompt command!
Or alternatively, you can just play the game online :)