Tried to offline a tablespace, but it failed with ORA-01539.
SQL> alter tablespace example offline;
alter tablespace example offline
ERROR at line 1:
ORA-01539: tablespace 'EXAMPLE' is not online
To figure it out what status of the tablespace currently is, we can check it by this.
SQL> select status from dba_tablespaces where tablespace_name = 'EXAMPLE';
It's already OFFLINE, no need to offline it.