Hi there,
We suspect it could be due to a plugin or theme conflict as currently, we aren’t able to find the required JavaScript file which handles the form submission on your website.
Will it be possible for you to do a conflict test on your site with only Email Subscribers and the WordPress default theme active?
You can also install Health Check & Troubleshooting plugin which eases the testing process by allowing you to deactivate other plugins and themes only for you without affecting other users or visitors on the site.
If the issue is not appearing after deactivating other plugins, conflict could be the cause. Reactivate them one by one and check which one is causing the issue.
Let us know if the issue still persists even after deactivating other plugins and theme. We will need to debug it further.
Thank you.