I'm trying to flag this as a duplicate, but a little red box pops up giving me a 404 error.
Is this a bug or am I not allowed to flag as duplicate (yet?)?
This Meta SE Bug Report figured out it was an issue to do with privileges. The developers removed the option when the user doesn't meet the required privilege level:
if a flag type requires commenting privileges (as off-topic flags do) and the user doesn't have them, that flag type will not come up.