There's a support thread with some recent entries on the BUTT SourceForge page here. The suggestion there is to run the following command in Terminal:
xattr -cr /Applications/butt.app
Then, try opening the app via LaunchPad. Let me know if it works for you.
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And FYI, once you get BUTT working you might also need JackPilot to capture the audio stream from your hardware. I'm no expert on it, and configuration will vary depending on your setup, but if you need help I might be able to point you in the right direction.