You need to add the key to your Bitbucket account manually. From https://circleci.com/bb/fleetonomy/main/edit#checkout :
1. Open Inspect Element in your browser's developer tools.
2. Click Create User Key
3. Find the new public key generated in the Network tab.
4. Go to your Bitbucket account and add it manually.
Sorry for this inconvenience. It is a Bitbucket specific issue that is being addressed, but not fixed.