The LAMMPS reader creates a multi-block dataset, with the unitCell (enabled by default), and the mesh (disabled by default). You presumably enable reading, and creating the “mesh” object as well. A vtkPolyData mesh is created with N cells and N vertices (for N atoms) with the scalars fields attached to it. Note that the uniCell has no data attached to it, and that is why you see the error.
The easiest way to read your data and use the calculator without errors would be to disable the reading of the unitCell, and only enable the “mesh”. Or to read both, but use an ExtractBlock to only select the “mesh” object, before passing it to the Calculator.