hi dev77,
Welcome. I too just am getting started with Grav CMS and come from a background of working in Wordpress.
I think the error is caused because Quark has many more templates it uses. Installing Quark skeleton first created all the pages, like the modular pages and blog and posts pages and I don't think Mediator has templates for those pages.
You could manually change the page types or create the missing twig templates but it is a bit of work from what I can tell and probably not good for just starting.
What I find easier is to use Theme inheritance ( like creating a child theme in Wordpress ).
Install your Quark Skeleton or which ever you like and then create a child theme to make modifications. Just be sure to use the Devtools to create the inheritance theme.
You can set your theme back to quark manually by editting /user/config/system.yaml
change:
pages:
theme: mediator
to
pages:
theme: quark
https://learn.getgrav.org/16/themes/customization#theme-inheritance
Maybe one of the experts can give some feedback what it the best approach to dealing with missing templates. Copying from one theme to another theme all the missing templates I would think would make a mess of the site.