Abaqus nodes are missing degree of freedoms. ErrNodeMissingDofConstrDef

Problem Description/Question: 

I model a cantilever beam with 2D plane stress elements. 

The left end of the beam is clampled (encastre).

In Step 1, I use a coupling constraint U2=coupled to apply a vertical load on the right (loaded) end of the model.

In Step 2, I use a different coupling constraint U1=coupled to apply a traction load on the loaded end of the model.

Each constrain is defined with different reference point, different surface, totally independent of each other. 

I get this error: "nodes are missing degree of freedoms. The MPC/Equation/kinematic coupling constraints can not be formed. The nodes have been identified in node set ErrNodeMissingDofConstrDef".

What can I do? 

PS. I cannot find "ErrNodeMissingDofConstrDef", where is this supposed to be? 

Required information to reproduce or answer: 

Attached: noConstraintDisable.txt > change extension to .py > run from CAE with Menu: File > Run Script, or copy paste the content to the Python window >>> below the workspace. 

Workaround:

The Workaround is to run 2 separate models, one for each load, but that is holding subsequent development of the model where I need the output of both steps to be available during a single execution.