You must be in 'Edit' mode, in 'Edge Select' mode

Try making your model bigger (at least bigger than 2 meters, but bigger is always better).

Check if your model has holes (non-manifold).

Make sure your model has length...solid squares and sphere shrink to a single point.

Make sure your model has volume - if it is planar, extrude or solidify it.

Check your normals - 'overlays...face orientation'

Try disabling 'cache' in the preferences.

Check your normals - 'overlays...face orientation'. If you mesh has red faces, flip those normals in Edit mode with 'Alt+N...flip'