ORA-08178: illegal SERIALIZABLE clause specified for user INTERNAL

Error
Tried with SYS user, got the following error:

SQL> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE NAME 'KK';
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE NAME 'KK'
*
ERROR at line 1:
ORA-08178: illegal SERIALIZABLE clause specified for user INTERNAL


Solution:
Tried with any user rather than sysdba user:

SQL> CONN TEST@XEPDB1
Enter password:
Connected.
SQL>
SQL> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE NAME 'KK';
Transaction set.