I've run into this a couple of times. Typically it's when starting a new project and gradually adding dependencies. By default, when adding (poetry add <dep>),

I've run into this a couple of times. Typically it's when starting a new project and gradually adding dependencies. By default, when adding (poetry add <dep>), poetry adds the most recent version of the library, and in pyproject.toml that becomes the minimum possible version. If you then later try to add something that depends on that library, but isn't up-to-date on the most recent version, poetry will (correctly!) error out because it fails to find a compatible solution. For example, something like this: poetry add ingredients # most recent version: 1.0 poetry add cake # requires ingredients, but <1.0 So here, poetry would add ingredients to your pyproject.toml with version = "^1.0.0" in the first step. In the second step, when you tried to add cake as a new dependency, it would say it can't find a version of cake compatible with your pyproject.toml's requirement for ingredients. The solution is to change pyproject.toml to be more permissive about the version of ingredients, resulting in it downgrading to a compatible version. Or submit an issue and/or PR with the maintainers of cake to bump up the supported versions.