So you say you have an existing btrfs filesystem on /dev/sdb, yet you tell LXD you want to create a new btrfs pool? That doesn’t really make sense.
What you want to do is tell LXD to configure a new storage pool, choose a name, say it’s btrfs, then say no to creating a new pool, at which point it’ll ask you about the name of the existing pool or dataset, which in your case would be /media/adrian/btr-store/lxd or something along those lines.