lpp page in Github does not exist.

I have the same problem,does anyone know how to fix it?

How exactly does this interfere with installation? LPP is just a post-processing tool ...
Anyway, instead of the (now vanished) CFDEMproject version you may use a fork such as our version at
https://github.com/ParticulateFlow/LPP
with a recently added python 3 compatible version at
https://github.com/ParticulateFlow/LPP/tree/python3

Another possibility is to directly write VTK files from LIGGGHTS using dump custom/vtk (requires compiling against the VTK library) instead of dump custom

Which directory would it be advisable to install the lpp?

There have been many reports that LPP no longer works reliably on modern environments.
LPP was developed for Python 2 and is based on the older Pizza.py toolkit; as a result, it often runs into compatibility issues and bugs on systems where Python 3 is the default (e.g., Ubuntu 20.04+).
If your goal is converting LIGGGHTS dump files to VTK, I have developed a new program in Python 3:

• Open source (GitHub): https://github.com/opendemjapan/D2V-dump2vtk.git
• Windows binary installer: https://opendemjapan.booth.pm/items/7611221

Highlights

• Includes an easy-to-use GUI; can also be used as a CLI for batch workflows.
• Converts not only particle data to VTK, but also force chains.
• Supports force-chain analysis using the Louvain method for community detection.

Please give it a try and feel free to share feedback or report issues on GitHub.