Missing Android platform(s) detected: 'android-27'

For anyone get this issue, you should uninstall Android SDK 27 then re-install it from SDK download manager.

@TrongHiue's answer didn't work for me. (Mac user, Android 3.2.1).

I had to close the IDE, go to Library/Android/sdk and rename the 'sdk' folder to 'sdk2'. Then reopen Android Studio - it will prompt you to tell you it can't find the old sdk folder but will automatically use sdk2. Hit ok. Then, let Android Studio re-index. It will now find platform 27 and you should be able to launch your app.

If you want to change your sdk folder back from skd2 to sdk, close the IDE, rename back to 'sdk', re-open the IDE, update the SDK location in File -> Project Structure, and make sure local.properties is pointing to 'sdk' folder and not 'sdk2'.

In my case, I fixed it by click "Edit" (Next to Android SDK Location) and install SDK platform at SDK Setup window.

Click here to see image