Unable to Disable www Prefix

Hi Community,

As the helpdesk of Squarespace is a bit busy (haven't received any reaction in couple of days) I try my chances in this space, hoping you can help me. 

I want to connect my website to a third party. I need to remove the www prefix as it is a subdomain. The issues I'm having is that I can't toggle the button. I tried a different browser, disconnected the domain and reconnected it again multiple times. I'm out of options now.

Anybody knows how to solve this problem?
Have a nice day!

Troy