I think the problem is in the trajectory (dcd file)for POPC. I had POPC in water, so I needed only the trajectory for POPC. Maybe my method of selecting only the lipids was wrong, because I got a mismatch in the number of atoms from psf file for lipids and the trajectory file for lipids.
Do you have a recommendation of how to obtain the trajectory file for POPC only from the trajectory file of POPC in water?