Is there a way to rebuild schematics outside of creative mode? I'm trying to fill caves but every time i try to look at it from outside in spectator mode i have to get closer and go to creative mode and then build it.

I'm not sure what you mean exactly?

Does the Rebuild mode not work in Spectator mode? What do you mean by look at it from the outside? And what do you mean by "I have to get closer and go to creative mode"?

So what exactly are you trying to do? If I go by the description as I understand it, you have a schematic of something that includes caves, and you want to use the Rebuild mode to edit the schematic to fill in the caves in the schematic?

If this is the case, then I think doing it in a creative world would be the easiest way. You may be able to use spectator mode or Tweakeroo's Free Camera to do it as well, but I think both of those would have issues with holding/selecting the replacement block, as you can't really easily change or control what you are holding in either of those modes.