Why can't I add an edge loop? I'm trying to add an edge loop on the first raw of the vertical squares. Why doesn't it appear on the last one and how can I fix this? Thanks.
Edge loops work only in clean quad topology, when they reach a triangle or a N-gon (your case) they stop. To add one more edge to your loop you can use the knife tool (shortcut K), then select the last vertex of the loop and click where you want the final vertex.