There is a known issue in sheet metal with the Add Closed Corner tool where the Closed corner will not work on both sides of a bend in certain cases. Typically you will find that you either cannot select the other side of the bend, get a message that a closed corner is already applied, or the result applies to the opposite side of the bend. The reason is that one of the bends that will be referenced in the tool has been flipped. For example, You drop a bend and then use the arrow to flip it or you edit the angle and type in -90.  To verify the case, you can edit the angle and if it is a -90, you will have the issue. To address it, re-drop the bend so that it is oriented in the correct direction on drop. 

This issue is known and will be looked at for IronCAD 2020 PU1 to be resolved.