Create 3D-printable Object With No Capped Sides

I have a “frame,” and a tab it’s standing on, as seen below that I’d like to make available to 3D print. I would only like the frames of the figure and box to be 3d printed and not the entire object as a solid.


Currently, I’m using the AddPipe command to create a surface so that the frame looks like:


However, when I try to export it as an .stl file, I get the following error message:

“No surface objects to export”

Could anyone give any insight on this matter?