This issue can occur when Lab Management is configured with SCVMM 2012 R2 Preview 

Workaround : 

Repair the Lab Environment once. It should fix the problem. 

Cause: 

While creating a network isolated environment Lab creates a virtual network using SCVMM. This network creation completes
successfully but SCVMM still returns a failure to Lab with the error message “The given key is not present in the dictionary..”
This results in Lab operation failure and puts Lab Environment in creation failed state. Since during repair the network already
exists, Lab Management doesn’t try to re-create one and thus the Lab Environment creation is ends up successful.

This is a known bug on Lab Management 2013 Preview and SCVMM 2012 R2 Preview