If you are using the “any platform” version of the simulator client (required for Linux and Mac users, optional for Windows users), you will need to make sure that you have the .net core 3.1 runtime installed.
Previously, the client was using .net core 2.2.
If you run into any issues, first try deleting your simulator client and re-downloading the latest version from the site. If you still have an issue after trying that and updating your .net core version, post here and we can troubleshoot.