Your server and storage nodes also take up a Networker client license but I don't think that is the issue here. These storage nodes are "dedicated storage nodes" which means they can only back themselves up which is why the licensing cost is much less... When you create the device for that storage node ensure you check the box for "dedicated storage node" and you should be able to add it.

Every time I need to configure Dedicated Storage nodes, I do a maneuver:
1 - Add the licenses of Dedicated Storage Node.
2 - Add a trial license for Storage Node
3 - Add and configue a Storage Node.
4 - In devices this Storage Node that you just added, check the option Dedicated Storage Node.
5 - Now the Storage Node license is free.
6 - Repeat step 3 and 4 until all Storage Nodes add.
7 - Delete the Trial Licenses

If you set the Dedicated Storage Node attribute when the device is created you can avoid having to do this!

Thanks all Fixed