With my human priest I am occasionally told that there are no valid targets for my attacks when there is an enemy standing in the red attack area. If I then play with another character or skip a turn it will let me attack the same enemy just fine the next turn. I would chalk it up to me not understanding other circumstances, except that as far as I can tell nothing at all changed with the priest or the enemy each time, yet I was able to attack them on a subsequent turn. This is frustrating when you carefully plan out a move and it tells you that you can't do it. If anyone has another idea as to why this is happening please enlighten me, otherwise I'm convinced it's a bug.
What specific card are you using (or various - if it's occured more than once) ? You're not under the effect of a stun et c? It sounds like you can play the card - but not select the targets?
It's either been Simple Bash or Bludgeon every time. No active effect cards on me when it has happened. I'll need to take a screenshot next time it occurs to see if someone can straighten me out. I've seen it about 3 times in my last 4 hours or so of playing.
A screen shot would be valuable for determining the problem; It's entirely possible a trait such as Squeamish was overlooked.
That is weird; most of the usual suspects would prevent you from clicking the attack card instead of giving a message. In fact, I think only Taunt is capable of giving that message when an enemy is within range.