Try:
nsenter --mount=/run/snapd/ns/lxd.mnt /snap/current/lxd/bin/btrfs filesystem resize max /var/snap/lxd/common/lxd/storage-pools/default
Try:
nsenter --mount=/run/snapd/ns/lxd.mnt /snap/current/lxd/bin/btrfs filesystem resize max /var/snap/lxd/common/lxd/storage-pools/default