wp-polls.php results in a fatal error: undefined function add_action()

The ajax call results in:
Fatal error: Call to undefined function add_action() in /.../wp-content/plugins/wp-polls/wp-polls.php on line 33

In my theme there is:
ajax_url: "https://www.domain.com/wp-content/plugins/wp-polls/wp-polls.php

If I call the wp-polls.php directly, the same error.

Earlier versions worked fine. Google shows that many other websites have the same error, too.

Anyone a solution?

https://www.ads-software.com/extend/plugins/wp-polls/