ERR_Name_not_resolved error in multiple areas

I recently inherited the administrative role for a client account using a site with a WP subdomain. The structure was that the main site, mkdeltalandsgroup.com was coded as a separate html file and the subdomain mkdeltalandsgroup.com/subdomainA was the actual WP site. As I had to create a new subdomainB, it didn’t make sense to have a new WP install and then manually connect to the hard-coded html page because then the client and I would have to manage 2 or more separate WP backends to make edits.

So I spent some time to recreate everything using multi-site functionality. So now the backend is connected to the main mkdeltalandsgroup.com domain and the 2 subdomains can be handled through the WP admin backend. It was a bit of a struggle to get this all working live from my localhost development.

However – now the client wants a subdirectory to use as a sandbox. When I’m in the admin backend and I go My Sites > Network Admin and navigate to anything on that flyout, I get an error:

‘This webpage is not available
ERR_NAME_NOT_RESOLVED’

This error is also encountered if you are logged out and go to https://www.thisismydomain.com/wp-admin and go to ‘forgot password’.

(I realize that it’s simpler to create a sandbox using a subdomain like dev.mkdeltalandsgroup.com but that still wouldn’t address the issue that new subdirectory sites cannot be added)

I have no idea how to fix this because it’s by pure luck that I got multisite to work at all. I’m not a developer by any stretch of the imagination and what I’ve read in other forums makes no sense to me ?? Any advice?