MacOS Server won't connect to a Server on my LAN

I want to manage a server on my LAN using macOS Server and it will not connect. I have tried the machine's IP address, the host name on the LAN, with the Admin name and password. It says "Connecting to Server" then the dialogue box "shakes" as if the password is incorrect. It is correct. the What is the secret?

I am using a 13" MacBook Pro with Catalina - updated and Server App - 5.9