Ran into an interesting situation today. My opponent had Han Solo and Shoot First in hand. Neither of us had Ranged damage dice in the pool. He wanted to play Shoot First just to trigger Han's ability. Is it legal to play a card if you can't complete the effect? We couldn't find anything in the rules which specified either way.

Shoot first allows you to resolve dice, thus.

RESOLVE DICE (p14):
Each die side has a symbol on it (see page 9). A player may resolve one or more dice in their pool that have the same symbol, one at a time (unless adding a modified die, then the dice are resolved simultaneously). To resolve a die, a playermust pay any costs and carry out the effect represented by the symbol on that die. Then they return it to the card that it came from.

A player can only resolve dice in their own dice pool.
A player can resolve dice with different values during the same action, provided the dice share a symbol.
A player cannot choose to resolve dice symbols if they have no symbols of that type to resolve. A player must resolve at least one die when taking this action.
A player can resolve any dice with the same symbol, even if those dice were not showing that symbol when the player started resolving dice.
A player cannot resolve the same die more than once per action.

So your opponent needed a symbol on one of his dice to satisfy the highlighted point.

Now, if your opponent did have a symbol and resolved it as per Shoot First:

“THEN” EFFECTS (p17):
In order to resolve an effect that is preceded by the word “then,” the previous effects on the card must have fully resolved (i.e., the game state changes to reflect the intent of the effect in its entirety). If the part of an ability that precedes the word “then” does not successfully resolve in full, the part of the ability that follows the word “then” does not attempt to resolve.

At this point he could resolve the removal of your dice. Because he has resolved the effect that preceded the word "then" he can attempt to resolve the rest of the card, if you do not have any symbols then he cannot have you discard a dice. However that part of the card (like say Let the Wookie Wins dice/card removal) would be optional.

Is it legal to play a card if you can't complete the effect?

Is it legal to play a card if you can't complete the effect?

Yes, the effect does nothing but you get the ambush part.

This doesn't sound right. Can you point this out to me in the RRG? /salute

Editted to add this:

PLAY A CARD FROM HAND

Before a player plays a card, they should first check to see if there are any play restrictions on the card . If there are, and those conditions are not met, then the card cannot be played.

Maybe I confused cost with effect?

Playing "All In = Resolve any number of your dice in the order you want." while you have no dice is legal because you can choose zero. There is no cost.

Playing "Award Ceremony = Spot a Red character to draw cards equal to the number of exhausted characters you have." while not having a Red character would be illegal.

Ok, I see. You can play a card that that has zero effect as long as the Cost to play is met. Right dudes?

Page 16 Abilities. You are probably confusing "resolving a die" action with "playing an event" action. If you are still not sure, then read spotting page 19.