As a TO, I know this question will come up because of Composure. In the latest rules, they have added the following

• While acquiring a lock, it fails only if there is no valid object to choose

but nowhere does it state if an object that is NOT selected is an invalid one. The rule also does not explicitly state that you CAN'T choose to fail a Lock, unlike the Barrel Roll action.

• The player cannot choose to fail a barrel roll if one of the three positions would not cause the action to fail.

I can see a scenario where 2 ships are close to the edge of range 3. One is in arc, the other isn't. They measure to the one that is in arc and it is out of range, but the other that is out of arc is in range. Do you have to lock that ship? Or can you choose to fail the lock and take your focus from Composure? Keep in mind that the value of Composure drops to nearly worthless (though it may already be) especially when you consider it is VERY likely that obstacles will almost assuredly be in range to lock. If you can't choose to fail, does this mean you will have to lock an obstacle?

You have to pick something thats in range...

so if there are no enemy ships, no allied ships, and no obstacles then you can fail.

"LOCKING
Q: While locking, can a player not choose an object?
A: Yes, but only if there are no valid objects to select. While locking, a player
must choose another object at range 0–3 if able. Thus, acquiring a lock
can fail if there is no other object at range 0–3, but only fails under this
circumstance." -page 23 rules reference.

You have to lock something in range if you try and lock.

That's what I was looking for. Totally missed that at the bottom. Thanks!

What Icelom said.

If it helps, when reading the rules reference, remember that when a card or rule says to do a thing, the must is implied and only overrides by an explicit “may” (rules reference page 2: card interpretation). So in the description for Lock (RR page 12) it reads “Choose another object at range 0–3.“ That should be understood as “You must choose another object at range 0–3.”

That includes obstacles, friendlies and devices. (RR page 13). It would be nearly impossible to have nothing else at range 0-3.