What I suspect is that you are trying to add the pane at the right side of a pane group that is not added in a SplitContainer yet and this is not possible, because it cannot detect in which SplitContainer the new pane should be added. Only adding to the center of a pane group is allowed before the group is placed in a SplitContainer.
As I don't know what the goal is - to add the newly created pane next to the group or in it, I cannot give a concrete advise what to do. Because of the title of the thread I assume you need to add the pane in the group, so you just need to change the last line of code to RSCDockRight.AddItem(PnlVehicleStatus, DockPosition.Center);
Hope this helps.
the Telerik team