During backup command processing, an attempt was made to allocate the data set to be backed up, which is identified in message ARC1001I. The allocation routines determined that the required volume is not online, nor are any units available to which a mount can be requested.
The backup command processing ends. DFSMShsm processing continues.
Arrange to have a unit made available for your request, and retry the operation.
DFSMShsm