In my case the NFS server where I hold the catalog is dr.local, and the NFS export is /data/catalog, so I use dr:/data/catalog.
In your case you may need a valid hostname and an export you can write to.
"Name or service not known" indicates the hostname "dr" cannot be resolved, so maybe use the right IP or hostname of your NFS server where the catalog share is available. Also try to manually mount it with `sudo mount -t nfs ...` to make sure it's usable and writeable as regular NFS share (before XCP service is started). Normally it doesn't have to be manually mounted, but it' s a good idea to try and unmount before attempting to use XCP.