You have shell elements and when you plot a component of stress such as Normal Stress, you can plot the results from several Positions through the thickness: the Top of the shell, the Bottom of the shell or the Middle of the shell.

The Middle of the shell is the membrane stress.

The Top and Bottom have the bending + membrane stress combined.

You can insert a Membrane Stress result, but that only works if under Analysis Settings, Output Controls, you turn on General Misc.

Notice that unlike regular stress plots that are in a Global coordinate system, the Membrane stress plots in the Local Element Direction.  That means you must know those directions. Here is how you can plot those. You can also force the local element directions to align with coordinate system direction.