close button in pop up?

Close button shortcode doesn’t work, no button is displayed, only a little square that’s not clickable and has no text and is not centred. In the preview screen it just shows the raw shortcode.

Found this support thread where you suggested using a regular HTML button with special class: https://wordpress.org/support/topic/button-close-2/ – but clicking it does nothing.

Please provide the correct way to add a Close button within the pop up. Thank you!

The page I need help with: [log in to see the link]