When trying to create PDB snapshots in 18c I ran into the ORA-12754 and below is how I resolved by enabling “_exadata_feature_on” parameter.

[oracle@3d9c27cdf1af ~]$ sqlplus '/as sysdba'

SQL*Plus: Release 18.0.0.0.0 - Production on Wed Apr 24 15:23:54 2019
Version 18.3.0.0.0

Copyright (c) 1982, 2018, Oracle.  All rights reserved.


Connected to:
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
Version 18.3.0.0.0

SQL> show pdbs

    CON_ID CON_NAME			  OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
	 2 PDB$SEED			  READ ONLY  NO
	 3 PDB18C			  READ WRITE NO
SQL> alter session set container = pdb18c;

Session altered.

SQL> select snapshot_mode,snapshot_interval/60 from dba_pdbs;

SNAPSH SNAPSHOT_INTERVAL/60
------ --------------------
MANUAL

SQL> alter pluggable database snapshot mode every 2 hours;
alter pluggable database snapshot mode every 2 hours
*
ERROR at line 1:
ORA-12754: Feature PDB SNAPSHOT CAROUSEL is disabled due to missing capability
.

SQL> show parameter compatible

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
compatible			     string	 18.0.0
noncdb_compatible		     boolean	 FALSE

SQL> alter session set container=cdb$root;

Session altered.

SQL> alter system set "_exadata_feature_on"=true scope=spfile;

System altered.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.

Total System Global Area 1610612704 bytes
Fixed Size		    8896480 bytes
Variable Size		  520093696 bytes
Database Buffers	 1073741824 bytes
Redo Buffers		    7880704 bytes
Database mounted.
Database opened.
SQL> show pdbs

    CON_ID CON_NAME			  OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
	 2 PDB$SEED			  READ ONLY  NO
	 3 PDB18C			  READ WRITE NO
SQL> alter session set container = pdb18c;

Session altered.

SQL> alter pluggable database snapshot mode every 2 hours;

Pluggable database altered.

SQL> select snapshot_mode,snapshot_interval/60 from dba_pdbs;

SNAPSH SNAPSHOT_INTERVAL/60
------ --------------------
AUTO			  2

SQL> set linesize 200
SQL> col con_name format a10
SQL> col snapshot_name format a30
SQL> col full_snapshot_path format a80
SQL> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, full_snapshot_path from   dba_pdb_snapshots order by snap_scn;

    CON_ID CON_NAME   SNAPSHOT_NAME		       SNAP_SCN FULL_SNAPSHOT_PATH
---------- ---------- ------------------------------ ---------- --------------------------------------------------------------------------------
	 3 PDB18C     SNAP_2457449251_1006442953	2051964 /u01/app/oracle/oradata/ORCL18/PDB18C/snap_2457449251_2051964.pdb

SQL> COLUMN pdb_name FORMAT A10
SQL> COLUMN snapshot_mode FORMAT A15SQL>
SQL> SELECT p.con_id, p.pdb_name, p.snapshot_mode, p.snapshot_interval
FROM   cdb_pdbs p
ORDER BY 1;

    CON_ID PDB_NAME   SNAPSHOT_MODE   SNAPSHOT_INTERVAL
---------- ---------- --------------- -----------------
	 2 PDB$SEED   MANUAL
	 3 PDB18C     AUTO			    120

SQL> alter session set container=PDB18C;

Session altered.

# Create a manual snapshot
SQL> ALTER PLUGGABLE DATABASE SNAPSHOT my_snapshot;

Pluggable database altered.

SQL> set linesize 200
SQL> col con_name format a10
SQL> col snapshot_name format a30
SQL> col full_snapshot_path format a80
SQL> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, full_snapshot_path from   dba_pdb_snapshots order by snap_scn;SQL> SQL> SQL> SQL>

    CON_ID CON_NAME   SNAPSHOT_NAME		       SNAP_SCN FULL_SNAPSHOT_PATH
---------- ---------- ------------------------------ ---------- --------------------------------------------------------------------------------
	 3 PDB18C     SNAP_2457449251_1006442953	2051964 /u01/app/oracle/oradata/ORCL18/PDB18C/snap_2457449251_2051964.pdb
	 3 PDB18C     MY_SNAPSHOT			2055024 /u01/app/oracle/oradata/ORCL18/PDB18C/snap_2457449251_2055024.pdb