KeyError: Missing 'nexus' section from the configuration file
or not a complete URL in --repository.
Maybe you have a out-dated '~/.pypirc' format?
more info: https://docs.python.org/distutils/packageindex.html#pypirc

[distutils]
index-servers =
    nexus

[nexus]
repository = some_url
username = "{{ NEXUS_USER }}"
password = "{{ NEXUS_PASS }}"

twine upload -r nexus *