The Delta Stage underlying SCAD is not particularly cleanly defined at present, and does seem to error like that quite often. It is not just missing walls, the actuator at the back of your image is not released from the shell. If I try to build it, it does the same.
Sometimes these effects can be from caching in OpenSCAD. I have tried to build the Delta Stage body locally a couple of times, but I have not reliably succeeded. There is a thread on the Forum where another user has modified the Delta Stage, they may be able to help.
Do you need the delta geometry for your application? The cartesian Microscope has a much more robust SCAD codebase, and many improvements in the structure. Custom design based on that will be much easier.
Edit: This could be a solution Delta Stage main body Printing issue - #22 by samuelmcdermott