While changing the password of internal oracle users in oracle 12c , after it got expired, you may get error as below.
ORA-600:[kpdbModAdminPasswdInRoot: Not CDB]
SQL> connect system/oracle
ORA-28001: the password has expired
Changing password for system
Retype new password:
ORA-00600: internal error code, arguments: [kpdbModAdminPasswdInRoot: not CDB],
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Warning: You are no longer connected to ORACLE.
Apply the patch 16901482 on the respective ORACLE_HOME.
Reference ( Doc ID 1583225.1 )