That line will always form when a face isn’t flat. Sketchup will break faces into triangles as necessary.
You can soften or hide the line by using ctrl+eraser or shift+eraser.
One way to create it without that line…
Whatever technique you used to draw that side, you didn’t get all the corners into the same plane, so SketchUp split it into two triangles. Drawing things slightly out of plane is a very common mistake that often results from not paying close attention to the inference engine’s tips. As the prior posts illustrate, there are often also better ways to create a particular shape. It’s all part of the learning process, so please don’t get frustrated!