Dart DevTools showing "Connect to a running app"

Start your flutter app with --observatory-port option. For example,

flutter run --observatory-port=9200

Once the app starts, you’ll see a message in your terminal that looks like:

An Observatory debugger and profiler on Android SDK built for x86 is available at: http://127.0.0.1:9200/Swm0bjIe0ks=/

Paste the URL you got from running your app (http://127.0.0.1:9200/Swm0bjIe0ks=/) into the connect dialog to connect your app to DevTools.

Hope this helps.

In Run flutter in Terminal with port you want. It can 9200 or 9300 or 9301 etcflutter run --observatory-port=9200

On the logcat you will see message look like"An Observatory debugger and profiler on Android SDK built for x86 is available at: http://127.0.0.1:9200/Swm0bjIe0ks=/"

After that copy and past url (http://127.0.0.1:9200/Swm0bjIe0ks=/) to dialog connect your app to DevTools

In my case the above answers did not work due I had another problem: PATH: the enviroment variables. This was because I couldn't execute flutter command in the terminal at my folder app project , so enviroment variables at system level it's necessary and I was doing this at user level for Windows. So I run flutter doctor o whatever to see if alredy the path was working and then I used the answer of @Hayeong Choi.