Errors when trying to install adaptive theme on D8 from backend GUI method
First thing is you don't install it. 

All you have to do is make sure adaptivetheme is somewhere in the /themes/ directory.

Then download and install the AT Tools module and it's AT Theme Generator sub-module, then create a sub-theme via the UI (a tab in the Appearance section). The at_core base theme will install automatically when you enable your sub-theme.

Also, this theme will never show up on the Appearance page, it's hidden deliberately, because it's just a collection of scripts and libraries that support features in sub-themes. It's never intended to be interacted with via a UI, or be set as default etc.