It could be that you may not have permission to edit the intersecting segments, or that you are trying to create a roundabout with no other segments / roads that intersect it.
You have to lay out the entrances / exits first, and then create the roundabout overlaying them.
Thanks for the help, it was that I hadn’t created the roads leading in to and out of the roundabout/traffic circle. After I did I could create it quite easily…