This error may occur while you've provided localhost or 127.0.0.1 IP address as server address while logging in to the Management Center.
You should make sure that the address field is not specified as: "localhost" or "127.0.0.1" - this is the most common reason for problem with assinging devices remotely if the Management Center and Xopero Backup&Restore are installed on the same host.
In the address field, enter the IP address of the host on which the Xopero server is installed.
If there's still a problem with assigning the device, while entering the correct IP address, log into the client application using account, which you want to assign this host to. First log in will cause an automatic assignment of current device to user.