Although it seems that all surfaces are merged or closed, it does not have to be the case. To check this, you need to switch back to the Generative Shape Design environment, select the Boundary function and click on the surface.
The Boundary function will create a boundary around the open contour (highlighted in green). This will not solve the problem of open surfaces but will indicate which area of the model
If the surface is closed, the system will not create a boundary and it will show the following error: