After inserting the tape, go to the devices tab, right click the drive and select "Inventory". After teh job finishes,. does the info appear in Blue (Overwritable) or Black (protected)
if black, then you need to change the OPP (overwrite Protection period of the media set that the tape is part of so that the tape will be expired when it is due to be re-used
Or you can manually drag the tape volume to the Scratch Media set from the Media Tab