-15 either means that your License Manager (NLM) isn't running or a firewall is blocking it (or the dreaded IPv6 incompatibility issue.)
I have many fixes listed for this in my signature line.
For a Quick-Start Guide on configuring the Autodesk License Manager, please see my post for further information:
For more information about Network License Manager connectivity issues and IPv6, please see my discussion post here:
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.