Thank you very much for your patience, We are able to replicate error on our end with effective-emails-to-passive-candidates-course course.
Here is what we found.
There is no text coming for ok button , please see attached screenshot below. All css are applied inline and we guess this button is rendered using jQuery. If there is conflict with theme js then please try by removing js script from functions.php. You can look on puresimple_scripts() of functions.php. You can comment out wp_enqueue_script() for checking.for an example.