One element simple shear test

[RFTomson]

Hello,

I am trying to simulate a simple shear test on a clay layer using one element.
My model has one element, a square in 2D plane strain; I define 4 nodes [the corner ones] and a surface [the top surface]

The initial step has 2 BC:
1. the bottom nodes [left and right] are prevented from translation on both directions u1=u2=0.
2. the top nodes [left and right] u1=ux=0.

In the next step I propagate the boundary conditions and, in addition, I apply a pressure load on top surface.
Quite simple, you could say.
I mesh the model such that I have only one mesh element. And I want to run the current situation so I can continue with the shear stage.
I intend to have a confinement step followed by a shear one.

Still, the analysis gives me the following error.

Node set assembly_soil-1_soil-1-refpt_ has not been defined
Unable to locate node set assembly_soil-1_soil-1-refpt_. The node set must have been previously defined.
Node set assembly_soil-1_soil-1-refpt_ has not been defined
Unable to locate node set assembly_soil-1_soil-1-refpt_. The node set must have been previously defined.
1 elements have missing property definitions. The elements have been identified in element set ErrElemMissingSection.

I mention that I make an independent instance. Who exactly is this assembly_soil_ ref point?

What am i doing wrong? Why do I need a ref point and where?

Thanks!

Regards,
R.