[mtmail]

User initiated though moderators can also do it. Your account has negative karma and you don't see the 'flag' and 'vouch' links yet. https://news.ycombinator.com/newsfaq.html

[AnimalMuppet]

I believe that flag (and, I presume, vouch) takes more karma than "not negative", though I don't recall what the threshold is.

[gs17]

According to https://github.com/minimaxir/hacker-news-undocumented#flaggi... it's 31.