While I was trying to duplicate my database I got this error:

orcl_pri> rman target / auxiliary [email protected]_dr
 Recovery Manager: Release 12.1.0.2.0 - Production on Mon Apr 15 13:50:51 2019
 Copyright (c) 1982, 2014, Oracle and/or its affiliates.  All rights reserved.
 connected to target database: ORCL (DBID=XXXXXXXXXX)
 auxiliary database Password:
 connected to auxiliary database: ORCL (not mounted)
 RMAN> run {
 allocate channel c1 type disk;
 allocate auxiliary channel cr1 type disk;
 duplicate target database for standby from active database nofilenamecheck;
 }2> 3> 4> 5>
 using target database control file instead of recovery catalog
 allocated channel: c1
 channel c1: SID=21 device type=DISK
 allocated channel: cr1
 channel cr1: SID=1140 device type=DISK
 Starting Duplicate Db at 15-APR-19
 released channel: c1
 released channel: cr1
 RMAN-00571: ===========================================================
 RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
 RMAN-00571: ===========================================================
 RMAN-03002: failure of Duplicate Db command at 04/15/2019 13:52:23
 RMAN-05501: aborting duplication of target database
 RMAN-05609: Must specify a username for target connection when using active duplicate
 RMAN> exit
 Recovery Manager complete.

The problem is the I used OS authentication. The solution is to use password auth. :

orcl_pri> rman target [email protected]_pri auxiliary [email protected]_dr

 Recovery Manager: Release 12.1.0.2.0 - Production on Mon Apr 15 14:04:07 2019
 Copyright (c) 1982, 2014, Oracle and/or its affiliates.  All rights reserved.
 target database Password:
 connected to target database: ORCL (DBID=xxxxxxxxx)
 auxiliary database Password:
 connected to auxiliary database: ORCL (not mounted)
 RMAN> run {
 allocate channel c1 type disk;
 allocate channel c2 type disk;
 allocate channel c3 type disk;
 allocate auxiliary channel cr1 type disk;
 allocate auxiliary channel cr2 type disk;
 allocate auxiliary channel cr3 type disk;
 duplicate target database for standby from active database nofilenamecheck;
 }2> 3> 4> 5> 6> 7> 8> 9>
 using target database control file instead of recovery catalog
 allocated channel: c1
 channel c1: SID=35 device type=DISK
 allocated channel: c2
 channel c2: SID=1156 device type=DISK
 allocated channel: c3
 channel c3: SID=21 device type=DISK
 allocated channel: cr1
 channel cr1: SID=1140 device type=DISK
 allocated channel: cr2
 channel cr2: SID=15 device type=DISK
 allocated channel: cr3
 channel cr3: SID=1150 device type=DISK
 Starting Duplicate Db at 15-APR-19
 ........
 input datafile copy RECID=34 STAMP=1005660559 file name=/orcl/datafiles/data1_01.dbf
 datafile 35 switched to datafile copy
 input datafile copy RECID=35 STAMP=1005660559 file name=/orcl/datafiles/index1_01.dbf
 Finished Duplicate Db at 15-APR-19
 released channel: c1
 released channel: c2
 released channel: c3
 released channel: cr1
 released channel: cr2
 released channel: cr3
 RMAN> exit