Make sure that your Relay endpoint has the net command available. On some Linux distributions (such as CentOS 7.4), the net command is only being installed when installing the full Samba suite (Common + Client + Server). The net binary/command is usually delivered with the samba-client and / or samba-common packages.
The Relay endpoint must have installed the Samba package ( smbclient) version 4.1.0 or above, and the net binary/command to deploy Windows agents.