Tampa has 3.4m in cap space now. Karlsson makes 6.5m which means they need to clear 4m now (1m operational cap space). By trading away Callahan, they free up 5.8m, giving them 2.7m in cap space.
Next year, Karlsson will require a 5m raise to 11M AAV. Coming off the books are:
Girardi: 3m
Coburn: 3.7m
Stralman: 4.5m
Total: 11.2M
That alone would mean they have around 6-7m in cap space with 4 D-men signed (Hedman, McDonagh and Sergachev the others) and 5 forwards. Something has to give way for them to ice a decent team up front.
They can go for Karlsson, but I don't see them being able to keep Kucherov in 2019 if that happens. There's no way he signs for anything less than 8m with his numbers and there just isn't enough cap space to make that happen.