Note:
You cannot use the Round command to construct assembly-driven part features until you set the Assembly-Driven Part Features option on the Inter-Part tab of the Options dialog box.
Choose Features tab→Assembly Features group→Round 
.
On the Feature Options dialog box, specify whether you want to construct an assembly feature, an assembly-driven part feature or a part feature.
Select the edges of active parts to apply rounds to.
Note:
Inactive part edges are not selectable.
Accept the selection.
Finish the feature.
Tip:
Assembly driven features must be enabled. Click Solid Edge Options on the application menu. Then click the Inter-part tab.
Activate parts to apply rounds to. Edges cannot be located on inactive parts.
Variable radius rounds are not allowed for assembly rounds.
Soften corner is not allowed for assembly rounds.
Assembly driven rounds will become ordered features on the solid body in the selected component.