The drive being made offline is currently mounted with an open tape volume. If the transaction using the volume requires subsequent tape mounts and is unable to acquire a drive, the transaction will fail.
If this tape volume is one of a sequence of volumes that are required to process a transaction, a drive must be available for each mount. This may be any drive in the library, but if all other library drives are currently busy and are still busy at the time the new tape must be mounted, the transaction will fail.
If possible, make the drive online until the transaction using it ends. If there is an idle volume on the drive, dismount the volume first.