The tape should be labelled, but not checked in.
Also check your DEVCLASS (q devclass class8mm f=d), make sure the MOUNTLIMIT
is set to 1 or DRIVES.
Then take the tape OUT of the drive so TSM will see the drive is free.
Then issue your BACKUP DB command.
TSM will put a message in the ACTLOG (or the console, if you have one open)
telling you when to mount a scratch tape.
THEN put the tape in the drive.