Importing an STL 

• Select Insert → Insert Mesh

• Click the Center and Move to Ground buttons.

• Click OK

Generate Face Groups

From the Mesh tab select Generate Face Groups

From the pop-up, select your object, select type Accurate and click ok.

You should see a multi-colour object if things worked out.

Convert Mesh

From the Modify pull down, select Convert Mesh.

Select:

• Operation: Parametric
• Method: Faceted (Only option in the free version)

Click Ok.

Cleanup Faces by clicking on a triangle and deleting it. Fusion will try self-repair all adjacent planar faces resulting in a single face. 

Note: This does not work on any curved services.

You now have a solid object!

Reference