Changing blog address

Alright currently I’m working on a site where the set url and blog home is https://treatment4addiction.org

I want to change it to use the wwws so I go in to the options and set both the url and home to be https://www.treatment4addiction.org . When I save the settings the site errors and I end up getting the server’s default page (a Plesk “this site is coming soon” page) I recover by going into phpmyadmin and resetting the url and homepage to be https://treatment4addiction.org and the site returns to normal.

When I try to use a redirect plugin, it ends up causing a 301 loop.

What am I doing wrong?

— Trick