According to the godartsass doc:

Note: The v2.x.x of this project targets the v2 of the Dart Sass Embedded protocol with the sass exexutable in releases that can be downloaeded here. For v1 you need to import github.com/bep/godartsass and not github.com/bep/godartsass/v2.

You have to install dart-sass standalone binary instead of dart-sass-embedded. It's also important to not have JavaScript SASS (installed with e.g. npm i -g sass) version in a $PATH - you will also get the same error, because sass --embedded is not available in this version. You should be able to pass dart-sass binary location in godartsass.Options.

I was just struggling with this issue and finally realized what caused the problem.