How to concatenate Sass variables

Multiple issues here, the correct syntax is

$url: 'siteurl.com';

#some-div{
  background-image: url($url + '/images/img.jpg');
}

• When you assign a value to the variable, you need to use : and not =
• You should quote the image path as it’s a string.
• Remove the stray + symbol before $url

You can see the above code in action at SassMeister