Error with Displacement of Dynamic Group

I’ve downloaded lammps-28Jun14. I’m facing a problem to compute the displacement of a dynamic group and store it in a temp file. Here’s my code.

I've downloaded lammps-28Jun14. I'm facing a problem to compute the
displacement of a dynamic group and store it in a temp file. Here's my code.

how should this work? to compute a (per atom) displacement, you need
that group of atoms to be constant during the time you monitor it.

the error message already indicates that.

axel.