I checked the admin and I found and error in console (403 (Forbidden)) when trying to open Shortcode Manager dialog.
This means that something is blocking window to open. I see that you have installed some security plugins so this is probably the issue. I tried to disable them but it didn’t helped so probably plugins altered .htaccess or added code somewhere in theme.
Try to remember what did you installed last before this problem appeared and configure or delete that plugin and try to restore any changes that plugin might have done.
If you try to access file that renders Shortcodes directly, you will see the issue: