The Flag Posts help page says the privilege is awarded at 15 rep, which is true. However, there is no indication on that page that the ability to "flag to close" actually requires 50 rep. (See the answer to this question for the reason why.) This is confusing for users with more than 15 but less than 50 rep, and I think the help page should be edited to reflect the current UX.
According to this comment, only SE staff can update Help Center pages. How can I get this update request to their attention?