Following steps must be done for FlexDeploy Server and Endpoint where Git commands will be executed.

Locate or generate default SSH keys. If no key found generate using "ssh-keygen" command, if you accept defaults id_rsa and id_rsa.pub files will be generated in ~/.ssh folder.
Add SSH public key to Bitbucket account. You must use id_rsa.pub for this purpose. Note that Access keys only provide read-only access, so if you need read/write access, then load public keys in SSH Keys section.
Test SSH connection.
Configure SCM Integration instance.
1. URL Example - git@bitbucket.org:flexagonteam/libraries.git