Can't add purger

Same here, the add is there, which means purgers are available. The add button gets me to the list, but when I select and add a purger, I get a success message on the same list page without going through any configuration pages for the actual purger setup.

I should have done this to begin with, but clearing browser cache fixed this.

I just had the same issue, but I wasn't able to replicate it when I retried.

Make sure your browser's 'inspector' is opened, so that we can get a backtrace or reported error.

The callback to clicking the 'add' button is AJAX-based, so this is why the error fails silently.

I just experienced this. It happens when I try to save Varnish purger form in modal.

Details:
Core 8.9.13
Purge 8.x-3.0
Varnish purger 8.x-2.0 (not sure if it's up to this one)

Message:
An error occurred while attempting to process /admin/config/development/performance/purge/purger/d485c04f5c/config/dialog?_wrapper_format=drupal_modal&ajax_form=1&_wrapper_format=drupal_ajax: ajax.$form.ajaxSubmit is not a function

Same here. I was not able to add a purger. Finaly solved it...
But, please let me explain my situation since this was related to my nginx purge mechanism.
I have a location in my nginx server configuration in order to purge fastcgi_cache (with generic HTTP Purger module) like this:

location ~ /purge(/.*) {
     fastcgi_cache_purge phpcache $scheme$request_method$host$1;
     add_header X-FastCGI-Cache "PURGING";
}

This nginx location rule was the same as Purge's module settings url (admin/config/development/performance/purge) and i could not reach Purge's UI module configuration through "Add Purger Button". Nothing was loaded and no modal was appearing to set my options.

Hope this helps someone with the same issue.