If you are using the Acymailing module integrated with GDPR checkbox and you are not able to make the privacy checkbox a mandatory field, you must configure the GDPR as in the image below:
However you may receive the message 'Please enable the Javascript to be able to subscribe' when submitting the form, as reported in the Acymailing forum:
https://www.acyba.com/forum/5-acymailing-how-to/69873-hide-message-please-enable-the-javascript-to-be-able-to-subscribe.html
In such case you must setup a template override for the Acymailing module and change the default value of the 'task' hidden field from 'notask' to 'optin' as in the image below: