I looked around and found a ‘PackRotamersMover’ under ‘protocols.minimization_packing’ instead of ‘protocols.simple_moves’ – perhaps this is the method/module you are looking for?
You do not post or link to the ‘extend_terminus.py’ script – is it a publicly available script? More experienced users may be better able to help you if you give them more information about the script as well as the version of python, pyrosette etc.