I\'m currently using Web Essentials 2013 update 4 to provide support for compiling .scss files in Visual Studio.
The tool has worked fine for me in the past however
I had the same problem, and spent some time uninstalling and re-installing Web Essentials. I tried deleting the parts of AppData that seemed to be related to Web Essential, all with no success at compiling the scss file.
So I created a new .scss with minimal content, that compiled. Then I started copying over pieces from the failing one. After a couple of copies, the new one failed too. I was able to zero in on the failure. It was caused by this line:
$highlightcolor: #67a0f;
Do you see the problem? the hex color code is missing a digit. it should have been 67a0f5. Fixing this fixed the problem with the original file.
I had gone over the file looking for syntax errors, but didn't spot the short color def.
I hope this saves someone some time and frustration.