which package did you install on Ubuntu (e.g. NoMachine, Workstation) ?
does ubuntu server have to already have an XServer or Desktop Manager running
Yes, the XServer must be running to be able to connect to the physical desktop.
NoMachine (the free version) lets you access the physical desktop remotely and requires that the Xserver is running. You can also use NoMachine on a headless Linux server (without the need of having the X server running), more instructions are available here:
Any of the products from the Terminal Server range (e.g. Workstation or Terminal Server, https://www.nomachine.com/enterprise), allows to create virtual desktop sessions. In this case it’s not necessary to have the Xserver running.