Sorry I'm replying to such an old thread but seemed to be the most relevant one.
I'm rather new to 3D printing and still working my way around Cura, I am using a pre-made model for a 'Signal Box' assembly which must be printed in four separate parts, each of the cylinders are hollow in order for wiring to pass through them however I am getting errors in terms of red in the tubes which is obviously because it cant bridge them, but adding structures will surely fill the tube and ill be unable to remove them? And around the door of the power box the overhangs are rather detailed. Also, (as shown in my second image in layer view) some areas are greyed out? I've no idea if this means they wont print but they are different to the areas that are red? If someone could look at the images and tell me what I need to change, that'd be great. If you need anymore information let me know. The model was made to be printed and replies to its post have said it worked great without structures so I think it's something I've done.