gunicornの導入がうまくいかない

Question

こちらの手順に従い、AWSにおけるDjangoアプリの環境構築を行っていたのですが、「7,gunicornの導入」の部分でエラーが発生し、つまずいてしまいました。
pipenv install gunicornこちらのコマンドの実行結果は以下になります。

/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/attr/_make.py:618: RuntimeWarning: Missing ctypes.  Some features like bare super() or accessing __class__ will not work with slots classes.
  set_closure_cell(cell, cls)
Installing gunicorn…
Traceback (most recent call last):
  File "/home/app-user/.local/python/bin/pipenv", line 10, in <module>
    sys.exit(cli())
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/cli/command.py", line 254, in install
    editable_packages=state.installstate.editables,
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/core.py", line 1909, in do_install
    pkg_requirement = Requirement.from_line(pkg_line)
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 1068, in from_line
    if isinstance(line, pip_shims.shims.InstallRequirement):
  File "/home/app-user/.local/python/lib/python3.7/site-packages/pipenv/vendor/pip_shims/shims.py", line 254, in __getattr__
    return super(_shims, self).__getattribute__(*args, **kwargs)
AttributeError: '_shims' object has no attribute 'InstallRequirement'

こちらの結果からAttributeError: '_shims' object has no attribute 'InstallRequirement'こちらの行が原因ではないかと思い、調べてみたのですが、今ひとつ解決の方法が分かりませんでした。
エラーの解決のため、ご助言頂けましたら幸いです。