Ricardo,
What is the volume or tape number its trying to mount?
Look for it in the Media and Device Manager window, then click media, shuffle by media numeric value, then scroll down and see if you can find the offending volume, if you find it then look at the Robot type, Volume Group and Volume Pool. make sure that this tape is in the correct listings, for example.
robot type = TLD
Volume Pool = 00_000_TLD a "-" means the tape is not in the robot or not scanned into the database
Volume Pool = Netbackup
If all looks good try to eject the tape.
JD