The wms published in geoserver can't be displaying

Hello, I would like to ask you about the problem that the wms layer published by geoserver cannot be displayed. I started cesium on nginx, and the geoserver deployed under Tomcat and the corresponding CORS problem have also been deployed, but the tif data and shp data published cannot be displayed on global. I would like to ask where there is something missing. The following is my code. thank you!

function init() {
            viewer = new Cesium.Viewer("cesiumContainer", {
                // 添加WMS图层
                imageryProvider: new Cesium.WebMapServiceImageryProvider({
                    // WMS 服务地址
                    url: '',
                    // 定义wms图层
                    layers: 'demo:china_tif',