The project target (Android L (Preview)) was not properly loaded

Solution 1:

Please, note this Android icon in the upper side of the screen

Android icon

This icon is in the upper side of this screen:

Screen which displays the API version to render the code

Probably it's displaying the number 20. This is the selection to use the API 20 to render the window. Change it to 19, or 20W (not 20L). It'll fix the problem.

API 19 selected

Solution 2:

You may also need to modify eclipse config to run on Java 7 Runtime.

Add this to the beginning of eclipse.ini

-vm
C:/Program Files/Java/jre7/bin