I suspect your resource folder wasn't setup correctly, although I don't know IntelliJ very well, so I couldn't help you with that. I have added this thread to the IntelliJ forum. Maybe someone will comment on that.
The rule when finding a resource (if I remember correctly) is that if the path starts with a slash (/) the path is relative to the classpath. If it doesn't not start with a slash, the resource is relative to the current directory, usually the project directory.
You can use this debugging method to check what are the entries in the classpath for a resource name. You can change the logger references to System.out.println() if you like: