That looks like you included SASS code to your html.
If you may like you should check:
- In your
html file: did you really linkt the .css file to your html or did you link the .scss file to html.
- In your
CSSfile: is the SASS code compiled at all.
UPDATE: ERROR RECONSTRUCTED
What I did:
Added This code to a CSS file style.css ...
li {
border-bottom: 1px solid gray;
background: red;
a {
div {
border: 1px solid red;
}
}
}
Linked that to my HTML: <link href="style.css" type="text/css" rel="stylesheet">
... and got follwing error in Chrome:

RESULT: It is raw SCSS code which is includeded in a source which is marked as CSS so the browser is interpreting it as CSS
Note: Including as .scss file instead was not the same result.
I am not quite sure about the difference to your error which names <style> as source (in picture top right edge). But if you click on the source link in the browser it should lead you to the source.
Up from here is no chance to follow the error.
So please check your CSS sources:
Is the .css file the right one: maybe your are writing your SCSS in a CSS file which off course does not compile. That could happen i.e. when you setup the project and do a misspelling creating the SCSS file.
Maybe that code is added to the <style> tag in the head or body (added by a module or whatever) or is it marked as <style lang="css">
... or any other method to add/link css
Indeed best way is to check all your linked css sources.
That cannot be done up from here without more information :-(
Also possible but VERY ATYPICAL FOR THAT ERROR PATTERN: maybe check your SASS compiler once more ... if it is an unknown issue which blocks it to compile.