Hello. I went to Stream, and all my saved files now say incompatible. I'm unable to play previous saved games on Stream. Also, I wanted to download Chapter 9 for Windows, but it says I need a key.
Do you mean steam?
All save files previous to chapter 9 are no longer compatible, you can start from any chapter your want by using the chapter selection system. New game -> Start from chapter.
The version on steam is chapter 9 CE as well.
You will need to contact itch.io support and ask them about that, I don't know anything about a key. The game should be in your library.