The itch.io app throws the "Cannot read property 'build' of undefined" error when attempting to install.
I'm guessing here since I'm unfamiliar with the itch backend.
It appears that you are missing app manifests as described here https://itch.io/docs/itch/integrating/manifest.html which appears to be related to the itch upload tool, butler, described here https://itch.io/docs/itch/integrating/quickstart.html
This may be a step you just forgot to do, since the previous version of The Fixer worked with the itch app. Or, of course, I could be completely off here.
No big deal, I suppose. Direct downloads will always work.