Aura is specific to the champ, they just have to have it.  Then for it to apply, that specific champ has to be in the leadership position, or slot #1.  Then, you must be aware that some aura skills apply in all battles, but many apply only in specific content, e.g. in arena battles, or in dungeon battles, etc.

i

Apothecary has an aura skill.  To use it he must be in the leader slot, and in a dungeon.

When battling it keeps saying i havent selected a aura skill character.

i

They have to have an aura, AND they have to be in the leadership position.

You remind me of Madden with his telestrader on NFL games.  You have skill.   lol

As has been said:

Champ has to have an aura, be in leader position but some are conditional to specific content

Also, some champions require ascension to unlock their aura too.