Problem with extruding 2D Shape in Inventor
right click on sketch > edit sketch
you will notice that you can move the lines apart (no constraints) so click on them and move them apart a little bit to able to choose the end points.
activate the coincident constraint commend
click on end points to join them.
your final result should be fully constrained, if you try to move any line the whole tingle will be moved.
Ready to extrude!