Ok my guess is that highlighted edge is just extruded from the middle or the equivalent near the outside edge.
SO it is not connected like a loop cut would be to the othe redges it appears to be going through.
Put simply you can not make it into a loop cut through those face.
Over the top those faces probably will loop cut, Ctrl R.
But it will definitely stop above the area where the round headlight is in a face.
But you could connect the two verts that must be there where the image selected edge goes through and just J join between those two.
Below that will probably loop cut as the with the top.
Loop cuts only go through Quads. More or fewer vert constructed faces and loop cuts do not work.
Extrusions are new geometry not cuts through other existing geometry.