A backup or restore is in progress and the the session is in a MediaW (MEDIA WAIT) state; however the needed tape is loaded in the library and is not mounted; sufficient mount points are available. The backup/restore session is not mounting the needed volume. It is obvious the volume is in the server's in-use list.
Now to determine, which session is has the volume in it's in-use list, run: SHOW VOLINUSE
The item of interest is the Storage Services Sessions number. Then run: SHOW SSSESSIONS
From the output of this, you will be able to determine the session that is locking the tape volume.