Error message when adding a new site

Hi there,
I try to add a new domain to my multisite
http://www.com360.agency/wp-admin/network/site-new.php?action=add-site
And I get this error: "Missing or invalid site address."
The site I try to add is: "lovely.apartments"
But: if I type "lovely.apartments" in my browser as URL, the browser loads something as if this site now is part of my multisite, please try it, and the browser directs to:
http://www.com360.agency/pro-site/?action=new_blog
How to add this domain? Is it because of its ending .apartments ?
Thank you
Monika