"$ flutter build web" is throwing the error "Missing index.html"?

It would help to see the output of your flutter doctor, but I am guessing that your project is not configured for web.

If you haven't already, you need to switch to the flutter beta channel like so:

$ flutter channel beta
$ flutter upgrade
$ flutter config --enable-web

At this point, running flutter doctor should show that chrome is available.

Then, switch into your application root directory, and run

$ flutter create .

This will rebuild the project to support web.

Then, from the same directory, you can run flutter build web

I had this problem. I could run web app in debug mode but I couldn't create web app.

Run this line in terminal to solve problem:

 flutter create .

This will add web app to your project.