No Ports Available when trying to associate a floating IP

I've deployed Openstack using RedHat's Packastack allinone option on Centos7. I then followed the instructions at [https://openstack.redhat.com/Neutron_with_existing_external_network](https://openstack.redhat.com/Neutron_with_existing_external_network) to use my existing external network. Next, I created a private network for the admin project with a range of 192.168.13.0/24. I then started up a guest in the admin project, and it was allocated an IP from this range (192.168.13.2). However, when I come to associate a floating IP with this guest so that I can access it internally, I am receiving a message saying "No Ports Available". I am able to select an IP address from the pool that I setup when following the document above.  Can anyone advise what I have to do in order to complete this setup and map floating IPs to my compute instances on the private network please ?