Module not found: Can't resolve (build works locally, fails on Vercel)

What it means in plain English

The build can't find a file or package your code imports. On Windows/macOS, Header.tsx and header.tsx are treated as the same file; Vercel's Linux servers treat them as different.

Likely causes

• The import's capital letters don't match the real filename (most common, per Vercel).
• You renamed a file by changing only its case and Git never recorded it.
• A package was installed locally but isn't in package.json.
• A wrong relative path (../ count).

How to fix it, step by step

1. Vercel → the failed deployment → Build Logs → find the Module not found: Can't resolve '…' line.
2. Compare that path letter-by-letter with the filename in your GitHub repo (not your file explorer).
3. Fix the import or the filename so they match exactly.
4. If Git didn't pick up a case-only rename, run:

git config core.ignorecase false
git mv header.tsx header-tmp.tsx
git mv header-tmp.tsx Header.tsx
git commit -m "Fix filename casing"

5. If it's a package name, run npm install <package> so it's added to package.json, commit, push.
6. Redeploy.

How to check it worked

• The build log should finish with a successful deployment.
• Open the page that uses the component.

Sources

• https://vercel.com/kb/guide/how-do-i-resolve-a-module-not-found-error (opens in a new tab)