To proceed, you must have two or more distinct pieces of unattached sheet metal geometry (unattached walls) in your model, indicated by the icon in the lower right of the screen.
1. Click Model > Editing.
2. Click the arrow next to Merge and then click Merge Walls. The Merge Walls menu appears.
3. Select the base wall surfaces to merge with the unattached wall.
4. Select the unattached wall surfaces to merge to the base wall.