What's above the "BUILD FAILURE" line of your build output? For example, if the path to cmd.dir is wrong, I get a message stating
Could not load definitions from resource com/sencha/ant/antlib.xml. It could not be found.
Then I get the error you're seeing. Check your path for cmd.dir.
Code:
<properties>
  <cmd.dir>path/to/Sencha/Cmd</cmd.dir>
</properties>