I edited your post to remove the red text - it is not needed for emphasis (red text comes across as annoying and is difficult to read).
The problem is with line 11 in your build.xml file. If we don't see that file, how can we help? Also post the full command line you sued to run
ant. You might also use the -v option and post the full output.
The error message is especially puzzling because the presumed missing class is part of Ant.