ERROR: Module 'app': platform 'android-28' not found. and Design editor is unavailable until after a successful project sync

So, I am new here just installed Android Studio and then wanted to create a basic activity. But the thing is they want me to install the SDKs and Haxm which is not being installed through SDK manager. I have tried a lot and have used all the ways stated in Stack-overflow to get rid of this but none of their ways has functioned.

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How do I fix design editor is unavailable until successful project sync?

Just you have to do >> open File --> Sync Project with Gradle Files and then after >> open File --> invalidate Android Studio caches / restart so done this problem.:) Show activity on this post.

Go to Tools->sdk manager->install API Level 28 then restart.