The project target (Android L (Preview)) was not properly loaded
Solution 1:
Please, note this Android icon in the upper side of the screen
This icon is in the upper side of this screen:
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.
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