If not installing it as a plugin, you can install acorn with composer. Actually it’s recommended to install it with composer (bedrock or sage), and that’s what is officially supported. So, it kind of is a requirement, kind of not. I have had it working fine through a plugin though.
Have you made any edits to sage?
Were things working at any point and now it is not or have you not gotten things working yet?
This illuminate error can show up for lots of reasons.
Try these things:
- install acorn as an mu-plugin
- see this post, starting at the bold word “Bonus” for instructions to add debugging to Acorn: Critical error when disabling WP_DEBUG in Roots Sage 10 - #14 by folbert