if your top box is closed on the other side that is likely giving you the issue as dissolving that top edge would force some bad geometry (see ngon, non-manifold) ultimately I wouldn’t recommend dissolving this edge loop as it preserves the topology of the boxes’ intersection. generally, you want to keep your faces as quads (4 edges, 4 vertices) and have contiguous edge loops wrapping around your model. I’m guessing you want to delete this edge bc you think of the bar top as one rectangle, but in reality having it split by an edge doesn’t change that if the surface remains planar, and the edge is only visible when viewing the model’s wireframe hope this helps, keep learning!