Hm that's strange, gui-butler is supposed to store it's own credentials, and setting an API key manually via environment variables is not something I expected or explicitly supported.
From your screenshot, it looks like it's stalling during the login call, which is surprising since there is a try-catch there and I'd expect it to at least give a generic error popup.
The project is based on electron, so if you'd like to try building from source, you'll need to:
- Install node/npm: https://nodejs.org/en/download
- Open a terminal inside the project folder
- run `npm i` to install the various dependencies
- run `npm start` to run in development mode
In development mode, the chrome devtools should be accessible and the console tab will hopefully provide more information about the stalled login.