If you are having trouble exporting an STL of your design, try these things:
1. Make sure you are on the Chrome browser (The native Windows Store app has a known export bug).
2. Try selecting the objects in the editor, then clicking Export to export only the selected objects. Often, this fixes a hung export. The reason is that there is a bug where some invisible object may be in the design that is causing the export to hang.
3. On a more complicated model, try creating "Parts" out of the repeating objects. Parts are flattened versions of your designs without all of the underlying grouping history to process.
To create a Part, you can select the object, such as a repeating column, then click "Create part" in the Shape Panel under category "Part Collection".
When you have used parts in place of the objects made of many nested groups, your scene will be optimized and easier exported.