Close loop. Sometimes when you try to extrude or revolve a sketch, you only can select lines instead of a surface. This is a very simple example, it just explains how you can close a loop. You can see that the sketch is not fully constrained in the beginning. That is the first thing you have to check. In the beginning that you learn Inventor all your sketches should be fully constrained. I' am sure there are other ways to fix an open loop.