Subassembly Composer can be a bit confusing sometimes. I’ve run into a problems creating shapes with them when one of the links uses the Surface Link from the Advanced Geometry Tool Box. The problem is caused by the Surface Link not being abled to use points to determine the start and end point of the links. Instead you provide the Start and End X values. You can do this via a Points X value or Offset Targets.
When you attempt to create a shape, it won’t work. It won’t work because SAC doesn’t think the points connect, even if they have the same X, Y values. In order to fix this you need to add a link between the Start Point of the Surface Link to the Point referenced in Start X, and the same for the end. Once the short links are connected for the start and end points then shapes can be created.