You may encounter this error when creating a curtain wall.
Error: No curtain panel families loaded. Cannot make arced curtain wall.
Issue: You are sketching a curtain wall and then try to unload it as you are creating it, or you sketch a curtain wall and with the wall tool active, you unload curtain walls.
Solution: If you unload curtain walls, you may be unable to draw another wall, until you reload at least one curtain wall panel.