ORA-01031 while creating a view as DBA
create table usr_t_user_reg
(username varchar2(30),
db_instance  varchar2(30),
primary key(username,db_instance)
)
/

CREATE OR REPLACE VIEW usr_v_user_not_reg AS
  SELECT username "User", db_instance "Instance",
    (
      CASE
        WHEN username IN
        (
          SELECT username
          FROM sys.dba_users
          MINUS
          SELECT username
          FROM usr_t_user_reg
        ) THEN 'not registered'
        WHEN username IN
        (
          SELECT username
          FROM usr_t_user_reg
          MINUS
          SELECT username
          FROM sys.dba_users
        ) THEN 'no longer present'
      END
    ) "Status"
  FROM usr_t_user_reg
  WHERE db_instance = 'TEST_DB'
    AND username NOT IN
    (
      SELECT username
      FROM usr_t_user_reg
      INTERSECT
      SELECT username
      FROM sys.dba_users
    )
/


Running the script with sqlplus gives the following error message
Table created.

          FROM sys.dba_users
               *
ERROR at line 8:
ORA-01031: insufficient privileges

this is one of the errors where the error description of the Oracle Database Error Messages 11g Release 2does not give a usefule hint

ORA-01031: insufficient privileges
Cause: An attempt was made to change the current username or password without the appropriate privilege. This error also occurs if attempting to install a database without the necessary operating system privileges. When Trusted Oracle is configure in DBMS MAC, this error may occur if the user was granted the necessary privilege at a higher label than the current login.
Action: Ask the database administrator to perform the operation or grant the required privileges. For Trusted Oracle users getting this error although granted the the appropriate privilege at a higher label, ask the database administrator to regrant the privilege at the appropriate label.

In the Oracle Database SQL Language Reference 11g Release 2 you can find the following prerequisite for creating a view

The owner of the schema containing the view must have the privileges necessary to either select, insert, update, or delete rows from all the tables or views on which the view is based. The owner must be granted these privileges directly, rather than through a role.

Neither SELECT_CATALOG_ROLE role nor DBA role is sufficient for creating your view.
So issue a
GRANT SELECT ON DBA_USERS TO your_user;

and now your view will work.
Instead of the object privilege SELECT ON DBA_USERS you can grant the system privilege SELECT ANY DICTIONARY. In  contrast to SELECT_CATALOG_ROLE this is not a role but a system privilege.
Note that the error message spots to line 8 of the CREATE VIEW statement that is the line that actually contains the object that causes the error.