If you are not using a Tape Device for your backup tasks, then you might be using disks?
Either way, try assigning a device/path to the SBT device before you perform your backup:
Start RMAN from your prompt
Connect to your target
RMAN> connect target /
Allocate a path you your SBT device (this is for a path device):
RMAN> allocate channel for maintenance device type sbt parms 'SBT_LIBRARY=oracle.disksbt, ENV=(BACKUP_DIR=<your backup directory>)';
Issue your backup command without the SBT device:
RMAN> backup tablespace <tablespace_name>;
I once read:
You should configure default devices and channels in advance of running RMAN Backup.
RMAN Backup | SS64.com