If I'm playing a tactic card whose the effect cannot be apply is this tactic card must be discarded or it must be returned to the person who played it?

For example: Favor of the guild thieves with runes everywhere on the map.

No, Corey has ruled previously that if there is no legal place to put the rune, the card cannot be played.

"the card cannot be played" yes, I read the FAQ too but so:

is this tactic card must be discarded or it must be returned to the person who played it?

"the card cannot be played" yes, I read the FAQ too but so:

is this tactic card must be discarded or it must be returned to the person who played it?

The card cannot be played. If someone tried to play it and then realized there's no legal position, the card would go back in their hand.

Agreed. If for whatever reason you try to play a card, but discover you aren't allowed to play it, it would go back to your hand. The rules don't allow for arbitrarily discarding cards; you can only do so if you actually play the card (legally), if you exceed 10 cards, or if some game effect specifically tells you to discard.

Thank you for all these précisions.

Agreed. If for whatever reason you try to play a card, but discover you aren't allowed to play it, it would go back to your hand. The rules don't allow for arbitrarily discarding cards; you can only do so if you actually play the card (legally), if you exceed 10 cards, or if some game effect specifically tells you to discard.

Ooops...I did this last night as I ha a tactics card that was unusable. Something that referred to a city, when there were no cities on the map...