A new render setup system is available that provides a modern approach to creating render layers, collections and overrides. This new approach is intuitive and simplifies the management of complex scenes for rendering, and is now the recommended workflow and the default system in Maya.
You can continue to open your legacy scene files and use legacy render layers. However, you must first switch Maya to the Legacy Render Layers system. To do this, open the Preferences window, navigate to the Rendering category, then select Legacy Render Layers as the Preferred Render Setup system and restart Maya. The Legacy Render Layers system and the new Render Setup system are different and separate and cannot be used together. You can only use one or the other.
If you attempt to open, reference, or import a legacy scene file while your Preferred Render Setup system is set to Render Setup, errors would appear indicating that the combination of the two systems is not supported. Likewise, if you attempt to open, reference, or import a scene with Render Setup nodes while your Preferred Render Setup system is set to Legacy Render Layers, errors would also appear in the Script Editor.
It is not necessary to switch systems, however, to batch render scene files when using render.exe via the command line. Note that this feature is not available for mayapy.exe.