A couple of students mentioned this error during a lab in the VMware NSX: Install, Configure and Manage class this week.
The issue is simple to create and remedy. Logical switches are assigned VNI (VXLAN Number Identifier) from a Segment ID Pool. If the Segment ID Pool is not configured before you try to provision a logical switch then you will get the “Unable to allocate an available resource” error. The resolution is to configure the Segment ID Pool before attempting to create Logical Switches.
Dennis Bray's Virtual Place 