In https://github.com/python/cpython/blob/master/Lib/ntpath.py#L76
def join(path, *paths):
path = os.fspath(path)

the path variable used directly in fspath method without checking is exist or not so it cause :

  File "d:\python\37\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "d:\python\37\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "D:\Python\37\Scripts\pipenv.exe\__main__.py", line 9, in <module>
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\core.py", line 697, in main
    rv = self.invoke(ctx)
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\core.py", line 1043, in invoke
    return Command.invoke(self, ctx)
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "d:\python\37\lib\site-packages\pipenv\vendor\click\decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "d:\python\37\lib\site-packages\pipenv\cli.py", line 217, in cli
    do_py()
  File "d:\python\37\lib\site-packages\pipenv\core.py", line 1703, in do_py
    click.echo(which('python', allow_global=system))
  File "d:\python\37\lib\site-packages\pipenv\core.py", line 125, in which
    os.path.join(location, 'Scripts'), command
  File "d:\python\37\lib\ntpath.py", line 76, in join
    path = os.fspath(path)
TypeError: expected str, bytes or os.PathLike object, not NoneType'. Make sure pipenv is on the PATH.

This is not a bug in ntpath. Raising a TypeError is a normal behavior when passed a wrong argument and I think the error message tells good info. You should figure out why pipenv passes a None to os.path.join.