I'm not certain the cause of the problem here, but this plugin fails if poetry 1.3.1 and tox 3.27.1 are selected in poetry.lock. This currently happens right now even if you let Poetry pick the poetry and tox versions. For instance, with this simple pyproject.toml file, right now I see the following:

$ poetry lock
Updating dependencies
Resolving dependencies... (1.2s)
$ poetry export

Dependency walk failed at virtualenv (>=16.0.0,!=20.0.0,!=20.0.1,!=20.0.2,!=20.0.3,!=20.0.4,!=20.0.5,!=20.0.6,!=20.0.7)

Here's the created poetry.lock file.

This may be related to #118.