When using the '/guild appoint "player name"' command, nothing happens. No confirmation, nor error message is given, and the player is not promoted. I've confirmed this happening with a couple of other guild commanders as well.

The only time no message is given is when you try to appoint someone who's not online at the time.
The player needs to be online for the appointment to happen.
Is this the case for you?

Yes! It is the case. Perhaps if a warning was issued stating such it would mitigate confusion.

Or perhaps the devs could figure out how to do this with an offline player

"Or perhaps the devs could figure out how to do this with an offline player"

that would probably be best.

"Or perhaps the devs could figure out how to do this with an offline player"

Yeah I'm pretty sure this isn't something they need to figure out, but was more of a design decision. Make a suggestion! I'm sure most would +1 it.

We released an update that allows lieutenants to be appointed when they are not online.