RubyMine Shows Cannot resolve import into pass/scss file

There is no guarantee that it will be fixed, but RubyMine's File menu =>Invalidate Caches/Restart=>Invalidate and Restart may eliminate the error on RubyMine side.

Also, as Unarist commented, I think the layout corruption is a different issue from RubyMine's warning (if it is a fatal error, the Rails screen should not be displayed either).
If it hasn't been resolved yet, I think it's better to post a separate question as layout corruption.

By the way, I have also done a Rails tutorial using RubyMine, but there were no errors.
The video (screencast) from that time is being released on YouTube.
In this video, I made custom.css.scss and @import "bootstrap"; but there are no specific errors.

Watch the video below around 12:30.

Chapter 5 Rails Tutorial 3rd Edition Coding Live Video - YouTube

It may take some time, but if you look at the screencast I made from the beginning, you may find something like, "Oh my gosh, this is not my procedure!"

For more information about the screencast, visit the blog below.

"Rails Tutorial 3rd Edition Coding Live Video" - give IT atry

That's all for your information.