You are going to run into some confusion with the model until you clean it up a bit because in SketchUp edges intersect regardless of what layers they use whereas in Autocad they don’t intersect unless you tell them to do so regardless of what layers they are on. The dwg has multiple edges using other than Layer0 and one of the layers is non-visible. That will cause intersections to happen for mysterious reasons! You can see some of the consequences in @mihai.s animation where he has to try more than one edge before the face forms. That’s due to intersections with non-visible edges. Unfortunately, the intersections are created as soon as SketchUp imports the geometry, so you will have to do a bit of work to clean things up. Fortunately, the model is still quite simple, so this isn’t a lot of work.
For starters, I’d explode the component that SketchUp makes from the import. It serves no further purpose and avoids the need to open it for edit.
Then I’d delete the layers using the layers panel. Select a layer and click the “-” button. Since there are edges using these layers, SketchUp will ask what you want to do. Choose to move the edges to Layer0. Do this for both of the layers.
Now do the process @mihai.s showed of tracing over edges to generate faces. Because all of the edges are now visible, when a face doesn’t form it will be more obvious why. There are several instances of parallel edges only about 1/2" from each other. Given the size of the floorplan, you have to zoom in to see these as two lines.
Edit: on a more complex model, there are extensions such as Eneroth3’s Find Faces that can automate the process, but they will still have issues until you fix up the unintended intersections.