There are certain limitations for fires:

  1. Water Tower should be complete,
  2. Fire Station should be in your city,
  3. The Water Tower quest in the To-do List should be completed.
  4. The Fire Station shouldn't be under the upgrade.

Keep in mind that fires appear randomly throughout the day, so you may have to wait before a fire ensues. If this doesn't help, force-close the app, open it again and wait for a minute or two. If there are still no fires, close the game and try to launch it later.

Note that fires don't appear when the app is closed or your device is in Sleep mode.