Sorry, I can't help you there, I'm not familiar with slicr. It's possible if slicr doesn't allow support material (or your printer doesn't have it), it's not allowing you to create the gcode because the bottom part does require support material. If you're not able to use support material, you could modify the bottom part so that the "floor" (with the vent slots) is solid and extends all the way to the ground. Then there shouldn't be any major overhangs and support material hopefully won't be necessary. The cutouts for the various connection might pose a problem as well, depending on your printer.
Basically, I designed this to be made with a machine that uses support material.