It sounds like the host you are trying to use as your cloud capacity (provider vdc capacity) is already being used by another vCloud instance (or thinks it is). If the host in your provider cluster was used before you will need to uninstall the agent from it using the vCloud instance that controls it (or re-install the host).
Each provider VDC you configure in vCloud should have a dedicated vCenter cluster of hosts. In a perfect world these hosts will be clean and never used before (re-install them) -- this makes sure they are only used and controlled by vCloud; saving troubleshooting problems in the future.
(That all said you can share resources; just takes a little more setup and planning)