In case someone gets the same error, I got an error in 0.15 when trying to load my save from 0.14.1, about ValuedParameter missing from the renpy/parameter.py file. I was able to copy the ValuedParameter class from the renpy/parameter.py file from the 0.14.1 version which I still had installed (lines 83 - 112 in the file) into the renpy/parameter.py file of the 0.15 version to fix the problem.