Is it possible in lammps to create a membrane that is impenetrable towards fluid?
For example, a box with a membrane in its middle that is filled with water.
Please guide me;
Many thanks;

If by membrane, you mean an assembly of lipid
molecules, then no, LAMMPS does not build
complex molecular systems. Other tools can,
and you can use their output as LAMMPS input.
See the LAMMPS tools directory for several
tools that do this, like ch2lmp, which converts PDB
files to LAMMPS input.