I don't know what is causing this problem but do the following below. The problem will be fixed by the time you are done.
1. Make sure that there are no spaces in your SDKpath. So create a folder called AndroidSDK in the C directory.
2. Copy your Android-sdk folder to the C:/AndroidSDK directory.
Now, your SDK path should look like this:
C:/AndroidSDK/Android-sdk
3. Update your Android SDK path in the Unity Editor.
Now, check if it works by trying to Build for Android again.
If it fails, continue from below
4. Download Unity 5.4 beta.
5. Re-download SDK then copy the file to C:/AndroidSDK/Android-sdk.
6. Go back to Unity and Update the path
7. Build the Project for Android again, it will show an error and ask you to update SDK.
Click on the "Update Android SDK" buttom. Unity will Automatically update your SDK. Your SDK should then look like something below. Your problem should be gone by now.
