I use a lot of the FORTRAN version of lammps and it did not have this fix wall command. I usually needed a wall for my slab simulations and what I do was create a sheet of atoms that are periodically arranged (so that I can have periodic boundaries along x and y and my walls are located at the z-axis) . I define the wall’s interaction potential which is usually truncated 6-12 LJ and put a fix to set the force and velocity of these atoms to zero (so that they will not move).
For your case, you can create a cylinder made up of atoms that are hexagonally packed so that it would be impenetrable and set a fix to these atoms so that the cylinder will not move. And, put your liquid crystals inside the cylinder.
By the way, I would define all the atoms coordinates at the data file.
You can't have a wall in x and have x still be periodic.
You can have a z-cylinder wall (which is really a wall
in x and y) and still have z be periodic.