Knife tool won't select edges I've looked everywhere but nobody has this issue, I'm using blender 2.8 and I'm trying to cut a plane model from a top view, but when I press k, select the edge, and then try to cut the inside, it deselects the edge and ONLY cuts on the inside If you need more details feel free to ask. 

I'm following this tutorial (https://www.youtube.com/watch?v=0lj643VmTsg) and when he gets to  this part of the video (8:10) I have the issue where it deselects the edge right after I start cutting the inside
An invalid edge can prevent the Knife tool from obtaining a lock on it. Usually because the edge is doubled, often from making an extrusion of a face, thinking it's been cancelled with a right-click (which only cancels the move, not the face-creation) and leaving the double face in place.  Your first step would be to select all vertices, and Remove Doubles.