how to show a pop-up/overlay window

i am trying to show an overlay containing the position of where the mouse is clicked. i referred to the example in openlayers official webpage here but when i run the code, the console.log message is displayed showing the coordinates of where the mouse was clicked, but the overlay or the pop-up window never shows up. please let me know what is missing in the code posted below

code:

private visualisePolygonsAsMVTTiles(siteID,threshold,visualizationOperationID) {
    /**
     * Elements that make up the popup.
     */
    const container = document.getElementById('popup');
    const content = document.getElementById('popup-content');
    const closer = document.getElementById('popup-closer');
    /**
     * Create an overlay to anchor the popup to the map.
     */
    const overlay = new Overlay({
        element: container,
        autoPan: true,
        autoPanAnimation: {
        duration: 250,
        },
    });
    closer.onclick = function () {
        overlay.setPosition(undefined);
        closer.blur();
        return false;
    };
    this.map.on('singleclick', function (evt) {
        console.log(evt.coordinate)
        const coordinate = evt.coordinate;
        const hdms = toStringHDMS(toLonLat(coordinate));
        
        content.innerHTML = '<p>You clicked here:</p><code>' + hdms + '</code>';
        overlay.setPosition(coordinate);
    });

.css:

.ol-popup {
  position: absolute;
  background-color: white;
  box-shadow: 0 1px 4px rgba(0,0,0,0.2);
  padding: 15px;
  border-radius: 10px;
  border: 1px solid #cccccc;
  bottom: 12px;
  left: -50px;
  min-width: 280px;
}
.ol-popup:after, .ol-popup:before {
  top: 100%;
  border: solid transparent;
  content: " ";
  height: 0;
  width: 0;
  position: absolute;
  pointer-events: none;
}
.ol-popup:after {
  border-top-color: white;
  border-width: 10px;
  left: 48px;
  margin-left: -10px;
}
.ol-popup:before {
  border-top-color: #cccccc;
  border-width: 11px;
  left: 48px;
  margin-left: -11px;
}
.ol-popup-closer {
  text-decoration: none;
  position: absolute;
  top: 2px;
  right: 8px;
}
.ol-popup-closer:after {
  content: "✖";
}

i found it out. i just did not add the overlay to the map as follows:

        this.map.addOverlay(overlay)