You said "alredy one LTO meddia in a drive" so to me it sounds like you are using a standalone tape drive, not a robotic one.
Anyway, that error (96) is because NetBackup can't find any available media that can be used for the backup job - that could be because of media type mismatch, the tape being frozen, wrong media inserted in drive (or NetBackup wants to use something else than the one loaded, for whatever reason)
Like J.Hinchcliffe said, you need to make sure the media type and drive type matches up - simply saying all verified is not good enough, sorry.
Run vmoprcmd and find out the media ID of the tape loaded on the drive.
Also check the drive type.
And check the storage unit type too.
/usr/openv/volmgr/bin/vmoprcmd
/usr/openv/volmgr/bin/vmquery -m MEDIAID | grep "media type"
/usr/openv/volmgr/bin/tpcofig -d
/usr/openv/netbackup/bin/admincmd/bpstulist -L | grep Density
Your tape / drive can be physically Ultrium 3 or Ultrium 4 or Ultrium 2 - but NetBackup does not care about physical type, it cares about logical type.
From the 4 command syntax above, all your media/drive/storageunit types need to match each other.
If vmquery output says "1/2" cartridge tape 2" then your tape drive should be hcart2 type, and your storage unit Density should be of hcart2 again.
Even after all these match, and although NetBackup is designed to use the mounted tape first (in the case of standalone drives) it is still possible that NetBackup is trying to use some other tape than the one already mounted - for example, if the media is suspended/frozen.
For checking on the media status, use the admin console GUI...
Hope it helps.
Abe