I have created the Campaign and List. I have the Users ready and when I click to Add Subscription, I get a blank screen.
I am using Joomla 3.9.13 and Using AcyMailing Starter 6.5.1.
I can't reproduce your issue. A blank screen might also be caused by a server error. The best advice I can give you is to rraise a suppor ticket through the contact form on Acyba's website: https://www.acyba.com/contact-us.html
Please be so kind to include a link to this thread and login credentials for your backend and ftp.
I opened a ticket and set the error reporter as per request. I have the site down until completed and I am able to reset the error reporter.
Please let me know when you are complete.