After adding and saving a VTL an 'Exceeded License' error message appears, even though there was still had enough total VTL capacity within that license.
The issue here was that NetVault had exceeded a license for an item and as soon as NetVault has exceeded on an item, then adding any device would not be allowed until the exceeded item was resolved.
You will be able to see what you are licensed for as well as what you have used and exceeded by in the Server license - capabilities and usage Report.
The Server license - capabilities and usage Report can also be viewed under NetVault Console - Administration - Domain Management - Right click on the NetVault Server and select Properties - Usage tab.
In this case the amount of slots that the customer had for their tape library was not included in their NetVault Server license key.
Once the new NetVault Server license that included the correct amount of slots was added; the VTL could then be successfully added.
If you notice that your license is incorrect, then you will need to contact the License Department directly on:
+44 (0)1628 518 501