I can not think of a reason other than full inventory for "You cannot obtain the copy of Rem's Tale, chapter 7." to be displayed, so my guess would be on the inventory space.

You say there should have been enough of it, though, so it's definitely a good thing you reported it here.

By the by, unless there is a new battlefield I don't know of, its name is 'Waking the Beast'. not 'Walking the Beast' (though it's definitely not unusual to see people call it the latter). :]