Help with using roSGNode?

I'm not sure what you're trying to do here.
Is this your /source/main.brs script?

It looks like you're trying to create a screen without a scene.
That won't work.

Can you confirm your project structure?

It should look something like this:
/manifest
/source/main.brs
/components/scenes/Main/Main.brs - All your scenes brightscript goes in here.
/components/scenes/Main/Main.xml - Calls Main.brs

Brightscript components are identified throughout the app via their defined name in the XML markup.

Here is how I think your code should look:
/components/scenes/Main/Main.xml

<component name="Main" extends="Scene">
  <script type="text/brightscript" uri="pkg:/components/scenes/Main/Main.brs" />
  <interface></interface>
  <children>
    <Label id="myLabel" />
  </children>
</component>

/components/scenes/Main/Main.brs

sub init()
  m.label = m.top.findNode("myLabel")
  m.label.horizAlign = "center"
  m.label.vertAlign = "center"
  m.label.translation="[0,500]"
  m.label.width="1280"
  m.label.text = "there is no stream right now."
end sub

/source/main.brs

sub main()
  m.screen = CreateObject("roSGScreen")
  m.port = CreateObject("roMessagePort")
  m.screen.setMessagePort(m.port)
  m.scene = screen.CreateScene("Main")
  m.screen.show()
end sub