ORA-23515: materialized views and/or their indices exist in the tablespace

SQL> drop tablespace TEST1 including contents cascade constraints;

*
ERROR at line 1:
ORA-23515: materialized views and/or their indices exist in the tablespace

Now you have to drop all materialized view in this tablespace to drop this tablespace.

SQL> drop materialized view  <MV_NAME>
 

Materialized view dropped.

Elapsed: 00:00:01.70
 

SQL> drop tablespace TEST1 including contents and datafiles;

Tablespace dropped.

Elapsed: 00:00:01.37
SQL>