It looks like you’re deploying both the frontend and backend directories in the same project. That might be causing the files to have a different path than you expected. Please check the deployment output to make sure the URL is what you expect. Steps to do that are described in the debugging guide from the previous comment.
You used the react tag on the post, but what’s used for the backend portion of the app? I know Express is a common one, and it sometimes causes problems since it was designed for traditional servers rather than serverless.
There’s article an posted about how to use Express.js with Vercel. I recommend following that guide is your project uses Express. If it’s using a different framework, I’m happy to share more relevant resources. Just let me know!