no compiler error, why wrong?

double curly braces is for declaration and initialization of collections in one statement like

var collection = new LinkedHashSet<String>() {
    {
        add("adding");
        add("more");
        add("elements");
    }
};
collection.forEach(System.out::println););

You however need to take care of the syntax What you did however is not that declaration and initialization in one statement but you created the collection. Then you added a block (the curly braces) and inside the block you created another block. Here you finally filled the collection with content. That of course is valid code and possible but it's not what the task was asking.