GPG failed to sign the data

Follow https://www.git-tower.com/help/guides/integration/gpg/mac

If everything fails, use GIT_TRACE=1 prefixed to the git commit command to try and see what git is actually doing

Key might have expired, run the GPG Keychain application to check, it offers to extend the key.

You must then export the new key, delete the expired key in your GitHub account, and upload the new key to GitHub. Unverified past commits then become verified.