You can achieve it by creating a sub-domain in Security Director and then move this one device to that new domain and give the required role.
How to create domain: https://www.juniper.net/documentation/en_US/junos-space18.4/topics/task/configuration/junos-space-domain-security-director-creating.html
Domain overview: https://www.juniper.net/documentation/en_US/junos-space16.1/platform/topics/concept/domain-overview.html
Apart from sub-domain there is no way in SD to give access to a particular device.
If this worked for you please flag my post as an "Accepted Solution" so others can benefit.