There are two reasons why ExtendSim would be installed on a device, but the ExtendSim application does not appear in Settings > Apps > Apps and Features:

• If ExtendSim was installed to the end user's account by an Administrator. In that case, the End User can launch ExtendSim (usually from the list of apps in the Start menu) and use it as normal. However, they will not be able to uninstall ExtendSim. The app will appear in Apps and Features when the Admin logs in, so the Admin can uninstall it.
• If ExtendSim was uninstalled while the application is open. In that case, ExtendSim will not launch correctly, but will instead give an error message about missing documents. Also, the two folders that contain ExtendSim files will have some files missing (the ones that weren't in use when the app was uninstalled). Since the app has been "uninstalled", just delete the two folders: Documents/ExtendSim and Program Files/ExtendSim.