Perhaps you have removed the checkmark that indicates your desire to take part in races. If you have changed your mind, simply check the box once again.

You have joined the neighborhood after the race has already started. Wait until the race ends, and take part in the next one.

24 hours before the start of a race, the Leader of the neighborhood can prevent you from racing by removing the checkmark indicating that you’d like to participate. You can ask the leader of your neighborhood why they chose to do that. You can also simply re-check the box, and take part in the races of your own volition. Please note that you need to check that box no later than 1 hour before the start of the race. Otherwise, you will need to wait until the next race.