The Penguins will have an option to perform a compliance buyout this weekend, which could relieve their current cap situation and remove the blockage around the Karlsson trade. At the time of writing, the Penguins are currently $3.2M over the salary cap.
In an appearance with NHL Network, Elliotte Friedman warned that Dubas doesn't like using buyouts, but would do so if it could make a Karlsson trade possible. With their buyout window ending this tomorrow , the Penguins will either need to make a cap-relieving trade or use their buyout window if they wish to move forward with acquiring Erik Karlsson.