Thanks for your help !
I’ve already tried and it didn’t change anything.
I’m using ipfire version 142, and my default python interpreter is python 3.8 
What is yours ?
There is my error message :
$ borg -h
Traceback (most recent call last):
File “/usr/bin/borg”, line 6, in
from pkg_resources import load_entry_point
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 3251, in
def _initialize_master_working_set():
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 3234, in _call_aside
f(*args, **kwargs)
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 3263, in _initialize_master_working_set
working_set = WorkingSet._build_master()
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 583, in _build_master
ws.require(requires)
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 900, in require
needed = self.resolve(parse_requirements(requirements))
File “/usr/lib/python3.8/site-packages/pkg_resources/init.py”, line 786, in resolve
raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The ‘msgpack-python!=0.5.0,!=0.5.1,!=0.5.2,!=0.5.3,!=0.5.4,!=0.5.5,<=0.5.6,>=0.4.6’ distribution was not found and is required by borgbackup