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I could see pretty-checkbox's developer last published 4 years ago.

Let's say the new module used popper-js@2.0 and someone who already had popper-js@4.0 as direct or sub-dependency in their project is more likely to face unmet peer dependency on using the new module.

Downgrading the version is not recommended due to possible conflict with other modules. A workaround would be to add resolutions.

Before doing anything, ensure there is no other version of react installed globally, delete your node-modules folder and package-lock.json file. Ensure your package.json dependency has react@15.4.0 and only add popperjs@^1.16.0 if you're sure that other modules in your project are not relying on later versions of popperjs.

Take a look at this article for a good explanation on peer dependencies

If there are other modules that needs other versions of popperjs then in your package.json you could add an additional property at the end like below,