If you joined a Neighborhood during an ongoing Derby, all you have to do is wait until that Derby finishes so you can participate in the next one.
Why does it work this way?
Neighborhood Derby opponents are selected randomly, so long as they have the same amount of members as you. So if you join a Derby in the middle of the race, your Neighborhood will gain an advantage over the others by having an extra participant.
Your best option in this case, is to switch Neighborhoods during the ‘pause day’. This way, you won’t lose your prizes and will be able to participate in the next Derby.
In the meantime, get ready for a new Derby! It’s just around the corner!