ant debug

Buildfile: build.xml does not exist!
Build failed

Solution

Run

android update project --target 19 --path /path/to/android/project

or, if you are in your project's root directory already:

android update project --target 5 --path .

target is the build target for your project. Run

android list targets

to get a list of all available targets, must be equal target in AndroidManifest.xml.