Ospd-openvas failed after every reboot or some time

Traceback (most recent call last):
  File "/opt/gvm/bin/ospd-openvas", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 3259, in <module>
    def _initialize_master_working_set():
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 3242, in _call_aside
    f(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 3271, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 584, in _build_master
    ws.require(__requires__)
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 901, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python3/dist-packages/pkg_resources/__init__.py", line 787, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'ospd-openvas==20.8.0' distribution was not found and is required by the application
``


can someone please tell me how to install ospd-openvas because i tried serval times or how to resolve this error . `