In general, Veeam One Reporter isn’t capable, by design, of collecting only part of information in case the root layer has been added. For example, if you add a cluster, Reporter will try to collect all data related to it, since partial information (about only one host, for instance) is unlikely to shed a light on cluster DRS functionality, etc.
For now as a potential workaround, you can try to create additional vCenter account that is able to see only one datacenter, and then add vCenter to VeeamOne console, using it.
Moreover, in VeeamOne v7 it will be possible to suppress aforesaid error regarding lack of license.
Hope this helps.