All I can tell you is that I tried logging in and out, restarting my computer, and repeated attempts, and could never get anything but "Invalid target". This is using an Xbox 360 controller under Windows 10.

As soon as I changed to using the keyboard, I had no problem completing the quest.

It would not surprise me to find that this is specific to Windows 10 and the Xbox 360 controller, as it's known that Microsoft's support for that controller on Windows 10 has some shortcomings.

I just wanted to let people know; a web search finds other people reporting this problem on other (non-Square Enix) forums, none with a solution posted.

I attempted to enter a bug for this multiple times, and each time I got the error "Text is too long". I checked, and there's nothing I've entered that exceeds the given limits. Finally I gave up, and just posted here.