Home and root subvolume is not system share. And another share is set to system share

I have a problem for subvolume.

The root and home subvolume is not system share.
So it can be deleted! And another two subvolumes are set as system share.

Actually I cannot delete the subvolume checked as system share.

[root@rockstor ~]# btrfs subvolume delete /mnt2/nextcloud-config
Delete subvolume (no-commit): '/mnt2/nextcloud-config'
ERROR: cannot delete '/mnt2/nextcloud-config': Operation not permitted

I want correcting this problem.
Please help me.

If it is need to check the log, please let me know which log is required.