I had a database with container configured and i was not able to create a common user because of name validity of container
SQL> create user rcat identified by password; create user rcat identified by password * ERROR at line 1: ORA-65096: invalid common user or role name
Easy workaround would be to set the hidden parameter “_ORACLE_SCRIPT” to true
SQL> alter session set "_ORACLE_SCRIPT"=true; Session altered.
Now user got created successfully
SQL> create user rcat identified by password; User created.