In CASA versions before 4.3 there is a problem with libxml on some Linux distributions other than RHEL 5 and 6, e.g. Ubuntu 13 and Fedora 20. The libxml version shipped with CASA does not work with other system libraries. You will need to do the following to your installation of CASA at the shell prompt (exact version numbers used below will need to be replaced by the ones found in your distribution).

On Fedora:

cd casapy-42.0.28322-021-1-64b/lib64
mv libxml2.so.2.6.26 origlibxml2.so.2.6.26
ln -sf /usr/lib64/libxml2.so libxml2.so.2.6.26

On Ubuntu:

cd casapy-42.0.28322-021-1-64b/lib64
mv libxml2.so.2.6.26 origlibxml2.so.2.6.26
ln -sf /usr/lib/x86_64-linux-gnu/libxml2.so.2.9.1 libxml2.so.2.6.26

In other words, the libxml2 version of CASA needs to be replaced by that of the system. Once this is done, stat CASA anew and run scriptForPI.py.