I’ve drawn a set of lines and now I see that they aren’t ‘fixed’ or on the plane. Is there an easy fix?
I was going to suggest Eneroth’s Flatten to Plane extension but I see you use a free version of SU so you may not be able to use it.
In general, if you want to do this kind of thing, it is best to start with a flat plane and draw on that to make sure everything is coplanar. You can always delete parts you don’t want later.
You also could try using a plugin named “Drop GC”. It is typically used to drop component or grouped items onto irregularly contoured terrain, but it seems likely that it could be used to drop non planar items to a flat surface.
No guarantee here, because I have not used the plugin for this exact purpose, and you will probably need to make selected lines into components in order to drop them to the surface plane. You can then explode those components and connect the disjointed lines on the same plane.
EDIT: Because you use SU Free, a web based version, plugins are not available to you. You may want to explore using SketchUp Make 2017, which can be downloaded here: Download All
Sometimes when your model gets too complex, SketchUp has trouble closing faces. I’d recommend making the rectangular array a group, then incrementally closing the face (from one side to the other) until you find the problem.