Well, the error message would indicate that the new Topic/Article (to which you're trying to assign a Containing Topic/Article; aka the one being moved) either:
- Already has a (parent or) child that would not work in the new position in the hierarchy or
- Would disappear from view because of the current filter settings.
You say it's a new Topic/Article, so I don't know what all has already been assigned, but I'd check:
Does it have any parents or children? (Containing Topics/Articles or Governed Content in the Relationships Pane on the right?) If so, remove them.
Do you have any searches or filters set? If so, disable them.
Then try it again and see what happens.