Disabled "apply changes" on Android Studio

Question

Problem:
Some of you guys who worked with Android Studio knows that we have to click "apply changes" if we want to apply the changes for our application.
My problem now is that this button is disabled/grayed out so that I can not even click on it.
Here is what I mean:

What I tried so far:
I tried the solutions of this links here but it did not worked, neither the "Invalidate Caches/Restart" nor "Sync Project with Gradle files":

• Apply Changes Button Grayed out: Android Studio
  
• Apply changes button in android studio is disabled(gray color)
  

Does anyone of you guys know what else I can try to do?

Answer 1

From Android doc you can see :

Apply Changes is only compatible with devices running Android 8.0 (API level 26) or higher.

Answer 2

This is called instant feature of Android studio. That allow you apply change instead of running app again. From android doc you can see that.

1. You must enable instant run to use it.

On Windows and Linux, click File → Settings In the Settings/Preferences dialog, go to Build, Execution, Deployment → Instant Run.

2. Your app must be running in device

If you want take advantage of instant run, first run it by green play button (shift + F10). If your app is not running, instant run is disabled.

3. You must have the SDK platform corresponding to the testing device installed to use instant run.

Let's say you are testing your app on a device/emulator running marshmallow(6.0). You must have 6.0 SDK platform installed to use instant run on that test device.

Answer 3

Change Build Variant to debug

View->Tool Windows-> Build Variants