To expand a little on Mani's reply -
When you move a newly labeled, or a new tape into a media set, that volume immediately takes on all the properties of the media set, including Overwrite Protection Period, as of the date/time that you moved the tape. So if the media set has an OPP of 7 days, and you move a new tape into that media set, you cannot write to it for 7 more days.
The solution is to go ahead and label the tape if you wish, or just inventory, if you wish, and leave the tape in the Scratch media set. When BackupExec writes the tape header, it will move the volume to the target media set by itself.