Three things to check here
Firstly has the tape already been added using the worng rule so its media id is U201L3. If it has, and it hasnt been used yet then delete it from NetBackup and re-inventory the library. If it has been used put it to one side until it expires after which you can delete it, put it in the library to be added correctly and then bplabel it from a command line to overwrite the physical header.
Next alternative is that is was previosuly known as having a bar code of CLU201. If this is the case then it wont add it as it wants to assign the same media id to a tape with a different bar code. In this case you need to go to the library GUI or interface and change you new library to use short bar codes (it reports back to NetBackup just CLU201 as the bar code). You would again then have to deal with any tapes that have been inserted in the new library - delete unused ones, put to one side any used ones ready for labelling.
Third possibility - when the old library dissapeared the tapes were still in the library. Go to the Media Section at the top of the Admin Console so that all media are listed. Select all of the ones with a slot number showing, right click and select "Move" then move to standalone. Once the above two options have been checked re-inventory the library so that it is properly up to date.
Hope this helps