vCOps 5.6 uses two different types of license keys. There's a Suite license and a Standalone license.
- vSphere 5.1 supports the use of either Suite or Standalone licensing.
- Prior to vSphere 5.1 you have to use a Standalone license.
Also, make sure that you are not trying to use a vCOps 5.0 key, which will not work in 5.6.