Try this

seer.entsupport.symantec.com/docs/273044.htm

the tape is not assign to any Volume pool ... :(

Hot or Cold Catalog backups in use?

If you did use or are using cold catalog backups you may need to remove the tape from the catalog config.

You say you can't expire the tape? (bpexpdate command)

It will need unfreezing at some point "FREEZING media id 5122L3"

Have a look at this T/N:
How to use a previous NetBackup catalog backup tape as a general backup tape
which intimates that once the tape is unassigned then you should just be able to re-label (unchecking the "verify media label" box).

Please explain which of the steps (as suggested in the TN that Gerald mentioned) you have tried and what the result was?
Surely the tape must be in a pool - even if it's in the Scratch pool?

Please post output of:
vmquery -m <media-id>
bpmedialist -m <media-id>

Unfreeze followed by label should work.

Please also confirm which version of NBU.

I had a similar problem in the past when we changed from 5 to 6 due to some changes in BP Vault.  My master runs on Unix but you should still be able to do this on about any host type, just how depends on your platform (I am not completely sure of Window's tape drive access methods.)

The reason for getting the error is to prevent overwriting catalog tapes needed to restore the system should you have a catalog failure.  Catalog backup tapes do not have the standard header and label setup as do regular backup tapes so NetBackup locks them down.  The only way I have found to clear them to be usefor is to blow away the header on the tape.  The last step below is what was mentioned in the seer article above but I like to ensure the header is really gone before trying to relabel the tape just in case since NetBackup has settings to keep some media for being overwritten unless explicitly allowed via the bp.conf settings.

On a Unix/Linux systerm I do the following:
1. Mount the tape in a tape drive.
2. mt -t /dev/rmt/<non-rewind tape drive path> rewind
    --> Make sure at the header of the tape just in case NetBackup read the tape already.
3. dd if=/dev/zero of=/dev/rmt/<tape drive path> bs=256k count=20
     --> This wipes out the first few blocks of data with 0s.
4. Run bplabel on the tape to readd the header.