IIS Express supports HTTPS for development, which is nice. You can use the iisSettings.iisExpress.sslPort option in launchSettings.json to set the HTTPS port. However this port must be in the range 44300–44399.

Otherwise your browser will be unable to connect to IIS, and you will not get a helpful error message. This behavior is documented, and the documentation also has a section on how to use any other port.