In Manage forums, settings, we have:

Enable quick reply:
If set to yes users get a quick reply box for this forum. If the global option for quick reply is disabled or the forum not postable to the quick reply box will not be displayed, even if set to yes here.

First "the forum not postable to" is quite awkward and ungrammatical. I suggest "the user cannot post in the forum". Whether a forum is "postable to" depends on permissions.

Second, the "quick reply BOX" is not what is or is not displayed. It is the Quick Reply BUTTON.