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