It seems that PREP command does not belong to Klipper.
Git version: 'v0.13.0-193-gbcd451095-dirty'
Untracked files: klippy/extras/AFC.py, klippy/extras/AFC_BoxTurtle.py, klippy/extras/AFC_HTLF.py, klippy/extras/AFC_NightOwl.py, klippy/extras/AFC_QuattroBox.py, klippy/extras/AFC_assist.py, klippy/extras/AFC_buffer.py, klippy/extras/AFC_button.py, klippy/extras/AFC_error.py, klippy/extras/AFC_extruder.py, (+14 files)
You may ask those authors.