I can't start a battle! Why?

It's not possible to enter the queue for battle 15 minutes prior the start of a maintenance break.
This was implemented to make sure that all players that are currently in battle can complete them normally, and that new challengers do not enter a battle that might've resulted in an error and troop loss because of maintenance start.
If you managed to start a battle while the server was preparing for maintenance break, don't worry: you still won't lose your troops!