Page Not found error, has index.html & not in subfolder

Hi Everyone!

I have tried manually deploying my react app, it has a build folder, inside that index.html too. But after deploying it says,
" Looks like you’ve followed a broken link or entered a URL that doesn’t exist on this site.

Back to our site

If this is your site, and you weren’t expecting a 404 for this path, please visit Netlify’s [“page not found” support guide] for troubleshooting tips."

How to fix this? I have tried other apps too. Exactly same problem.

PS : This is my first post in community. Sorry if it’s not the right category.

Thanks for solving, in advance.
Sushree

This might help: