I got this error when I moved a running Android project to another system.
Error says, Project ‘MyProject’ is missing required library: ‘E:androidadt-bundle-windows-x86-20130729adt-bundle-windows-x86-20130729sdkplatformsandroid-19android.jar’. The project cannot be built until build path errors are resolved.
A red exclamation mark also appears on project icon.
Steps to fix this error
Select project ‘MyProject’ in the Package Explorer and press [Alt+Enter] to open Project Properties.
Click on Java Build Path in left pane and Libraries tab on the right pane. Select android.jar library with the error icon and click on Remove. Finally click on OK.
This should fix this error.