If you are changing your site or just purchased your extension and you don't manage to make fullPage.js work, check out the examples provided on Github or see the Codesandbox demos provided for every method and callback in the documentation. (Like this one)
If, however, you have not changed anything on your site and you are suddenly getting errors on your page or fullPage.js broke your site, here's probably why:
You are using CDN and the link is automatically loading the latest version, forcing an unwanted update on your site that can lead to issues when the new update contains breaking changes.
You can read more about it and how to solve it in this other article.
You might also find these articles relevant: