I recently migrated a Content Management System from Create React App to NextJS in order to score some SEO points.
One of the challenges I faced was seeing these errors at compile time.

window is undefined or document is undefined

Window, and document are not available on the server. This is why you'll run into these types of errors if you are trying to access window properties or document.
In my case, I was persisting my authentication token to localStorage on the previous application.

To avoid running into these undefined errors at compile and build time, you can run a simple check.

if (typeof window !== "undefined") {

localStorage.setItem(key, value)

}

This basically tells your piece of code only to run when it's in the client environment where it can access window.

Keep hacking.