In R2020b, graphics support has been removed in favor of a more performant SSH shell-based QNX real-time operating system. This means that it will not be possible to visualize signals on the target screen anymore.
As a workaround, monitor the data and signals on the host computer by using:
1) Simulink Data Inspector (SDI) to observe signals that you have marked for logging inside your model or using Simulink Real-Time Explorer.
2) External Mode and a Simulink Scope block in your model.
3) Instrument objects to programmatically control streaming data from a real-time application to SDI, or to stream data to an instrument panel inside an App Designer app. Below is an example of such an app:
It will still be possible to connect a monitor to the target machine. A text-based status log will be visible on the target screen: