I can't use my Ember attack with Fennekin

What's happening is that the PP (Power Points) has run out on the move.
To fix that, you can:

• Use an Ether/Leppa Berry or Max Ether on the move
• Use an Elixir or Max Elixir on Fennekin
• Go to a Pokemon Center and heal

All moves have a limited amount of PP (Power Points), Ember has 25, which means it can be used 25 times before it needs to be recharged. Typically, the more powerful the move, the lower it's maximum PP. If a Pokemon runs out of PP for all of its moves, it will attack by struggling, which damages it's self as well as the opponent.

To regain PP, you have a few options: Ether or Elixer items. Ethers will restore the PP of a single move, while Elixers will restore PP to all of the selected Pokemon's moves. Bringing your Pokemon to a Pokecenter, or any other location that fully restores them will also work.

You can increase the maximum PP a move has by using PP Up items, or PP Max. However, these items are somewhat rare, and should be saved for more powerful moves that your Pokemon learns as you train it.

This is called PP or Power Points. They tell you how many times a move can be used. You must use a PP-restoring item such as an Elixir, which restores 10 PP to all moves. or Ether, which restores 10 PP for one move. There is also Max Ethers, which restore all of the PP of one move and Max Elixirs, which restore all PP to all moves.. You can buy these at your local Poke Mart. A Leppa Berry can also restore 10 PP. These can be found on Berry tree. You can also restore both your PP and HP at a Pokemon Center. To raise PP, a PP Up will raise the number of times you can move by one and a PP Max will raise it to the max. Hope this helps!