When you created the sandbox with a specific name, you need to add “tenant” details in the Launch.json page.
[2020-05-24 01:28:25.17] Could not download reference symbols. Please ensure that:
1. The correct server name and instance are specified in the launch.json file.
2. The correct application version is specified in the app.json file.
3. The dependencies are correctly specified in the app.json file.