Apple Remote Desktop Authentication Failure

I use Apple Remote Desktop to help some of my relatives with any Mac/User issues. I am preparing a (newer) Mac for one of those relatives. Both machines (mine and the new one for my relative) are running the latest version of Sequoia (15.1) and ARD 3.9.8. Despite being side by side and hard wired to the same ethernet network, I keep receiving an 'authentication error' when I try to connect. I have just created the new user on the newer Mac and know both the username and password. Remote Access is enabled. I can see the new machine in both bonjour and network scanning, but I simply cannot connect to it. I can connect to other machines configured in the same way. Suggestions?

Thanks.

Mark.