Are you sure you don’t have some other OPC UA server running on the port the ArSim is trying to open the OPC UA server to? Some software run OPC UA servers on the default OPC UA port of 4840 and if some other software has that already reserved the ArSim OPC UA server does not start.
You could check the “Connectivity” log from the logger to see if the OPC UA server is not able start properly. If there is a message stating that the OPC UA server could not be started you either need to stop the software currently using the port, or configure the Automation Studio project to use a different port for OPC UA.
This older thread might have some more information:
AS4.12 - mappView similation Connection to configured OPC UA server failed