ORA-01031: insufficient privileges when Toad 11.6 connect to an Oracle 12c database

Solution

Access to SYS.USER$ has been restricted in Oracle 12c (for oracle 12c  security , role, privilege, references doc  https://docs.oracle.com/database/121/DBSEG/E48135-19.pdf)

The issue can be resolved by granting access to the SYS.USER$ table to the related user /schemas:

GRANT SELECT ON SYS.USER$ TO <user>;

For grant the privilege login as sys dba

Sqlplus sys/sys_password@orcl  as sysdba

SQL> GRANT SELECT ON SYS.USER$ TO <user_name>;

Grant succeeded.

Reference Oracle Doc ID 1958777.1)