Update:
pipenv 9.0.0 has been released, which should allow you to use pipenv install -e . as expected.
Original answer:
pipenv install -e is buggy and has been fixed in master (pull request). It will be available in the next release, sometime after Thanksgiving.
Temporary workaround for now is:
pipenv shell
pip install -e .
After the release, you should be able to run pipenv install -e . similar to what you’d expect with pip.