Netbeans SASS support was written for the original Ruby SASS. Modern SASS implementations (Dart SASS et al) are based on libsass.
There is very limited/incomplete support for libsass already in Netbeans (command line flags are NOT supported – but it should compile). To implement the workaround:
1) Open your netbeans.conf file (/etc/netbeans.conf in the NB installation folder)
2) Add the following to the netbeans_default_options parameter: -J-Dnb.sass.libsass=true
3) Launch Netbeans, Tools -> Options -> HTML5/JS -> CSS PreProcessors and configure it to point to your libsass implementation (e.g. the sass.bat file in your dart-sass installation folder)