This issue can occur if the guest OS is Windows 2012 R2 Preview.
Install Agents for the Lab Environment once. It should fix the problem.
When the test agent is installed and configured, by default it enables firewall exception for tcp port 6910 in private
and current profiles. If the guest OS is Windows 2012 R2 Preview the firewall exception is enabled for private,
domain profiles, but the current profile is public. This results in test controller not able to reach to test agent ends
up in putting Lab Environment in “Not Ready” state..
This is a known bug on Lab Management 2013 Preview.