Solution:
As Toontrack solo is a free host application designed to be used in conjunction with our other products it does NOT require authorization for itself. Any request for authorization you may see on-screen will therefore invariably be for one of your other installed Toontrack product, as identified in the header at the top of the authorization screen.
Please review the Authorization video provided in this section for an overview of the process. Remember, again, that the ‘serial number’ and ‘authorization code’ referred to in this video is NOT for Toontrack solo, our free host application, but the primary product you are trying to use within it.