IDC31611I WHEN CREATING A DUPLEX PAIR THE {SECONDARY | ALTERNATE} DEVICE MUST BE OFFLINE

Explanation
A SETCACHE request which requires that certain
volumes be offline was unsuccessful.  The following requests require
the specified volume to be offline: SETSECONDARY (establish a duplex pair) requires the secondary
volume to be offline.
REESTABLISHDUPLEX (reestablish a duplex pair) requires the alternate
volume to be offline.

System action
Processing continues.

Programmer response
Vary the secondary/alternate device offline
(in ALL hosts attached to the subsystem) and rerun the command.

Source
DFSMSdfp