If you book a game, don't show up and aren't checked in by the Master, you get a No Show penalty: you lose the game's credits plus 20 credits.
How to avoid it
Cancel for free up to 10 hours before kick-off.
Between 10 and 2 hours before kick-off, cancelling costs the game's credits plus 10 credits.
Less than 2 hours before kick-off it costs the same as a no-show (game credits plus 20), but please still cancel: it frees your spot so the game stays full.
Marked as a no-show by mistake?
If you were there but weren't checked in, appeal in the app within 12 hours of getting the penalty. See How do I appeal a penalty?