Hi Monica, never mind about the other pre-booleaned version, I was able to repair your file and fill in the hole, I've attached the result here.

I'm not sure why your original one did not join together properly - looks like some sort of bug in the geometry library. But at any rate I was able to get that problem area joined by deleting the top planar face and constructing a new one in place of it. The new one then joined in ok with the side wall pieces.

Then to fill in the hole area I extruded up a circle from and trimmed it, and then used Join to join those pieces together to make a finished solid, it should now work ok for STL export.

- Michael