Tried to drop a temporary tablespace, but it failed with ORA-12906.
SQL> drop tablespace temp1 including contents and datafiles;
drop tablespace temp1 including contents and datafiles
ERROR at line 1:
ORA-12906: cannot drop default temporary tablespace
ORA-12906 means that the tablespace you want to drop is required to be the default temporary tablespace of the database.
To solve ORA-12906, you have to change the default temporary tablespace to another one.