Point your phone at the floor and move your phone around until the camera detected a surface. The 3D asset should appear on the detected surface.
Occlusion isn't working!
WebXR which does not support occlusion is the default AR mode set for web-based experience.
Change the default AR mode to Scene Viewer which does support occlusion by adding a metadata key named arMode with the value scene-viewer to the 3D model. See how to add metadata in the Data Page section of the documentation.