Trading for Erik Karlsson is a complicated process made even more complicated by the Ottawa Senators insisting that teams take winger Bobby Ryan as part of a deal. This has been a persistent theme since the trade deadline, when the Las Vegas Golden Knights almost acquired the duo. As far as I've heard, the Stars aren't taking Bobby Ryan back despite that likely being the easiest path to completing a deal, and they have good reason not to want any part of Ryan.
But they should strongly consider taking Ryan back if that's what it takes to get Karlsson.