Jekyll is not compiling sass files

Here is the preview of the image

As you can see in the image the main.sass file in the _site directory is the same file as in the _assets/_css/main.sass
But the file (_site/assets/css/main.sass) is not compiling into main.css

Do you have two lines of triple dashes at the top of the file? This indicates to Jekyll to process the file.

Next make sure you’ve changed the sass source directory in config.yml as appropriate.

Docs:

Are you using jekyll-assets plugin? Because if you don’t you should prepend by default your assets folder with an underscore, same thing for css folder.

Because all folders starting with _ will not be copied to the destination folder.

The default Sass folder is _sass.

This may be a bit confusing for newcomers.
Be sure to check example website to get how to organize your Sass files with Jekyll: https://github.com/jekyll/jekyll-sass-converter/tree/master/example

I didn’t understood this part: