Presumably it means it can't find it, which is odd if it can find everything else.
You can try checking the hash of chitin.key in the game's base directory to make sure it isn't corrupted. The SHA-512 hash of mine is:
F51CB6038F897CF6281FB3E64294E8EA7AF0A9ED2C4C47F138CBFD84250763B4DEEF62DF03C852C88B8592AF9D096ED011F507285FE4E8C84508BF0AD1E9F123
If you don't already have a means to generate file hashes, you can try this online tool - https://emn178.github.io/online-tools/sha512_file_hash.html. Simply drag and drop chitin.key onto the page to generate the hash for it and compare it to the one I posted above (note that tool produces lowercase hashes while mine is uppercase, but that doesn't matter).
Alternatively, this is the option I use myself - https://github.com/namazso/OpenHashTab/releases/latest Per the name, it adds a new tab to the Properties window in Explorer that will list hashes of the selected file, like so: