Gagan, welcome to the forum, Roon and its dedicated users are here to help where we can.
First, there is not much information here to help you. Is your Nucleus Plus the Core or is the Mac your Core, and what type of Mac and the version of the MacOS are you using? More information on your network also would be helpful; what is using WiFi and what is using an Ethernet cable connection?
Finally, if the Mac is a client, have you set up the correct firewall exceptions in MacOS per these instructions:
The more details you provide the more we can assist. Have you looked at this information to help get started or report issues: