ORA-00230: operation disallowed: snapshot control file enqueue unavailable

Backup job failed because RMAN cannot make a snapshot control file. The message stack is as follows:

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup command at 06/25/2017 22:48:44
ORA-00230: operation disallowed: snapshot control file enqueue unavailable

When RMAN needs to back up or resynchronize from the control file, it first creates a snapshot or consistent image of the control file. If one RMAN job is already backing up the control file while another needs to create a new snapshot control file, then you may see the following message:

waiting for snapshot control file enqueue

To determine which job is holding the conflicting enqueue:

SELECT s.SID,
       USERNAME   AS "User",
       PROGRAM,
       MODULE,
       ACTION,
       LOGON_TIME "Logon",
       l.*,
       'alter system kill session '||''''||s.SID||','||s.SERIAL#||','||'@'||s.INST_ID||''''||' immediate;'
  FROM GV$SESSION s, GV$ENQUEUE_LOCK l
 WHERE l.SID = s.SID
   AND l.TYPE = 'CF'
   AND l.ID1 = 0
   AND l.ID2 = 2;

You should see output similar to the following:

SID User Program              Module                    Action           Logon
--- ---- -------------------- ------------------- ---------------- ---------
18 SYS rman@testdb (TNS V1-V3) backup full datafile: c30000110 STARTED 25-JUN-17

After you have determined which job is creating the enqueue, you can do one of the following:

• Wait until the job holding the enqueue completes
• kill current job and execute backup again