Splines & Creating Forms
By default you cannot create a single closed loop with one spline created in Revit. I’ve included a few possible scenarios and what may be occurring below:
This will create one spline which cannot be connected to the origin point. You would need to create a second spline and snap to the origin and end point to fully close the spline. If a form is created while open it will be a surface, if closed it will be a solid:
3D Spline by 3D Points
The second option for creating a spline will allow you to get closer to a closed spline in one pass. However if you snap back to the origin reference point it will create 2 overlapping points which will result in the create form failing.