PROBLEM:

While creating or altering a user with default tablespace got below error.

SQL> alter user dba_raj default tablespace UNDOTBS1;
alter user dba_raj default tablespace UNDOTBS1
*
ERROR at line 1:
ORA-30033: Undo tablespace cannot be specified as default user tablespace




SQL> create user TEST_USER identified by TEST_user default tablespace UNDOTBS1;
create user TEST_USER identified by TEST_user default tablespace UNDOTBS1
*
ERROR at line 1:
ORA-30033: Undo tablespace cannot be specified as default user tablespace

SOLUTION:

As the error says, we can’t set an undo tablespace as default tablespace for a user. The can only set as a PERMANENT tablespace.
We can get the type of tablespace by using below query.

SQL> select tablespace_name,contents from dba_Tablespaces;

TABLESPACE_NAME                CONTENTS
------------------------------ ---------
SYSAUX                         PERMANENT
SYSTEM                         PERMANENT
TEMP                           TEMPORARY 
TEST                           PERMANENT
TEST_2                         PERMANENT
TS_CRM_EXTCH                   PERMANENT
TS_GGATE                       PERMANENT
USERS                          PERMANENT
UNDOTBS1                       UNDO ------ >>>> 

So just use a tablespace with PERMANENT type,

SQL> alter user dba_raj default tablespace TS_GGATE;

User altered.