Android Studio 3.0 Execution failed for task: unable to merge dex

For Cordova based project, run cordova clean android before build again, as @mkimmet suggested.

This error happens when you add an external library which may not be compatible with your compileSdkVersion .

Be careful when you are adding an external library.

I spent 2 days on this problem and finally it got solved following these steps.

• Make sure all your support libraries are same as compileSdkVersion of your build.gradle(Module:app) in my case it is 26.

• In your defaultConfig category type multiDexEnabled true. This is the important part.

• Go to File | Settings | Build, Execution, Deployment | Instant Run and try to Enable/Disable Instant Run to hot swap... and click okay

• Sync Your project.

• Lastly, Go to Build | click on Rebuild Project.

• Note: Rebuild Project first cleans and then builds the project.

Try to add this in gradle

    android {
      defaultConfig {
        multiDexEnabled true
        }
   }

Resolution:

Refer to this link: As there are various options to shut the warning off depending on the minSdkVersion, it is set below 20:

 android {
     defaultConfig {
         ...
         minSdkVersion 15 
         targetSdkVersion 26
         multiDexEnabled true
     }
     ... }

 dependencies {   compile 'com.android.support:multidex:1.0.3' }

If you have a minSdkVersion greater than 20 in your build.gradle set use the following to shut down the warning:

  android {
      defaultConfig {
          ...
          minSdkVersion 21 
          targetSdkVersion 26
          multiDexEnabled true
      }
      ... }

Update dependencies as follows:

     dependencies {
        implementation 'com.android.support:multidex:1.0.3'
     }

Again the only difference is the keywords in dependencies:

minSdkVersion below 20: use compile

minSdkVersion above 20: use implementation

1. I hope this was helpful, please upvote if it solved your issue, Thank you for your time.
2. Also for more info, on why this occurs, please read the first paragraph in the link, it will explain thoroughly why? and what does this warning mean.