There a two (big) step mising :
Firstly you need to look at the version of Ren'py used by the game. Secondly you must perform all the following step with the exact same version if you want to be sure that you'll not have errors.
Ren'py have a long history of backward compatibility, but it was never a total compatibility. Anyway lately it start to throw errors for really strange things, so even the use of the last version isn't anymore a guaranty of success.
