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.