I was trying to access a specific resource from my Servlet. Before that I created a system user, then added permission to that user to a specific node. But I think I did a mistake while configuring the user in system console User Mapper Service due to which I am unable to retrieve instance of Resource from ResourceResolver. Please find the screenshot for further info.
Try clicking the top row and see if that makes a different. Also for you Sling mapping - make sure that you are using the OSGi symbolic name. See this article for more details and a working example: