Trouble printing sketchup in 3d print

Hi, I’m a noob here and there’s something wrong when I 3d print sketchup models.

I made a long bolt with thread in sketchup, and the red boundary line I marked gets erased when I save it as stl file.

So the 3d print will fill up the void I need to put the nut in.
Any advice will be very helpful.

First thing I notice is that it looks like you have reversed faces on the inside. Would be good to see the image not in xray.

You can cheat and draw lines From inside edge to the outside edge of the Nut . . Looks like you did not get the Hole on center of the Nut for the Threads to be installed in it . . Or you did not get the distance right like told it to make threads longer then the part was . . Like making a hole and using Push Pull to go through and you get a bit on the bottom side of the part as it went past the bottom edge . .