Possibilities

Please add slug="xxx" to the [wps-forum] shortcode, where xxx is the slug of the forum. For example, [wps-forum slug="my-general-forum"].

Forum (choose) does not exist.
Please add slug="xxx" to the shortcode, where xxx is the slug of the forum. For example, [wps-forum-page slug="my-general-forum"].Please add slug="xxx" to the shortcode, where xxx is the slug of the forum.No posts
Please add slug="xxx" to the shortcode, where xxx is the slug of the parent forum.
Please add slug="xxx" to the shortcode, where xxx is the slug of the forum. For example, [wps-forum-page slug="my-general-forum"].Please add slug="xxx" to the shortcode, where xxx is the slug of the forum.No posts
Please add slug="xxx" to the shortcode, where xxx is the slug of the parent forum.