Issue
Your release phase step has failed and you want to know why.
Resolution
You can view release phase output via the CLI or Dashboard as described here: https://devcenter.heroku.com/articles/release-phase#checking-release-status-logs
For programmatically retrieving release phase output you can use the API as described here: https://devcenter.heroku.com/articles/release-phase#checking-release-status-programmatically