I have found the solution to error from here
app.get('/*', function(req, res) {
res.sendFile(path.join(__dirname, 'path/to/your/index.html'), function(err) {
if (err) {
res.status(500).send(err)
}
})
})
but I am not able to understand what I have actually write in that index.html file

i am using that build static in my backendhow are you using it? If you put the html file also in build, so your code should beres.sendFile(path.join(__dirname, 'build/index.html')react-scriptsso the index.html is underbuildfolder therefor, the server serves this html file usingres.sendFile(path.join(__dirname, 'build/index.html').