A few months ago I was trying to create an Android build for one of my Unity3D projects which used to be primarily for iOS. When trying to create a build, Unity showed this error:

Error building Player: CommandInvokationFailure: Unable to list target platforms. Please make sure the android sdk path is correct.

The error log also displayed a bunch of Android Java files it was trying to access. To fix this, double to check if the Android SDK path is entered properly in your Unity by checking Edit > Preferences > External Tools-tab > Android SDK. If that looks ok, go through the follow instructions:

  1. Go to https://developer.android.com/studio/index.html
  2. Scroll down to the bottom of that page and find Windows “tools_r25.2.3-windows.zip“. Download and unzip it.
  3. Remove and replace your folder “SDK” + “/tools”.
  4. Use the Android “SDK Manager” and upgrade to 25.2.5.
  5. Now everything works again in Eclipse, Android Studio, Unity3D!

Source: https://forum.unity3d.com/threads/unable-to-convert-classes-to-dex-format-android-issue.197061/