You may encounter this warning when a curtain wall with non-system panels joins to a roof, and the curtain wall does not have a rectangular shape.
Warning: Non-system panel families cannot be used for non-rectangular panels.
Issue: This problem might also occur if you create a non-rectangular wall profile and then change that wall to a curtain wall with non-system panels.
A non-system panel, such as a glazed or empty panel, cannot have a non-rectangular shape.
Solution: If your panel is simple, create an appropriate panel type derived from the system panel family. If not, try making the panel in the wall or roof rectangular, and then using a panel family of the desired (non-rectangular) shape. The wall will then conform to the non-rectangular shape of the panel.