If you use the Create Base Feature command first, the Loft command should become available from the Model > Create menu.
Let me know if you have further questions.
Thanks,
FYI, this is just a temporary limitation. We're working now to add Loft to the parametric modelling workspace, and you should see this added in a future software update.
For now, as Nathan suggests, the best way to do this is to create a Base Feature in your design, which allows you to freely use the full set of non-parametric modelling commands to build geometry, including Loft.
Thanks for posting about this!
Jake
Jake Fowler
Principal Experience Designer
Fusion 360
Autodesk