This indicates a basic communication error with the license server. You will have to confirm that the system where NetSim is run, is connected to the license server system by doing a ping.
Further, it needs to be ensured that the license manager process (rlm.exe) is running in the license server system.
If the issue still persists, you can check the firewall/antivirus to ensure that the ports used by the license manager is not blocked. (Refer: https://support.tetcos.com/en/support/solutions/articles/14000104444-how-do-i-use-netsim-across-a-firewall-what-ports-should-be-opened-)