So $LOCAL_BRANCH can be (almost) anything you want - it's going to create a new branch of that name. So you might want to call it 'pr_30_from' if you like. I'm assuming you were checked out on the master branch, which is why you get that error - that's about the one name you can't use.
Let me know if that works. Charles
PS. Just as a word of warning, you can only fetch from open pull requests (at least in Stash 2.9+). If '30' has been merged/declined you will no longer be able to fetch from it.