In reply to Abuzar Gaffari:
The uvmf_yaml module is packaged with the UVMF install and should be automatically picked up by the script. There are a lot of potential variables that could cause this, including the version of UVMF in use, OS in use, as well as the version of Python in use.
Filing a Siemens support ticket would probably be the best course of action to get this resolved properly.
Short term, adding $UVMF_HOME/templates/python to the $PYTHONPATH environment variable may help resolve this, although it should not be necessary.