"Module not found" error on deployment but build works fine locally

When I deploy my Next.js project to Vercel or many other providers, I get the following error:

Module not found: Can't resolve 'some module' in 'some path'

But npm run build works fine on my computer. What's wrong?

The most likely cause is that you're using a case-insensitive operating system (Windows, macOS) and you have a file named, say, SomeFile.js and you're trying to import it as somefile.js. This will work on your computer but not on Vercel or other providers, because they use Linux, which is case-sensitive.

If you host your project on GitHub or similar Git hosting services, go there to check the correct capitalisation of the file path, and use it. You may also want to look up how to rename the file with git so that your prefered capitalisation is preserved.

Personal opinion: This is why I exclusively use kebab-case for file and folder names now. It is probably not a popular opinion in the React community, but it's exactly because I faced this kind of problem before and want to avoid it in the future.