You probably don't have enough experience levels for whatever you're trying to enchant. Each enchantment has a cost, if it says 12, it costs 12 levels.
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You can enchant items with an Enchanting Table.
You can enchant bows, swords, picks, axes, swords, and all pieces armour.
Enchanted Book or Encantment Table.
Use an anvil in creative mode.
you must enchant a tool but it might not be fire aspect all the time.