Change to the jenkins user and run the command manually:
git ls-remote -h firstname.lastname@example.org:person/projectmarket.git HEAD
You will get the standard SSH warning when first connecting to a new host via SSH:
The authenticity of host 'bitbucket.org (188.8.131.52)' can't be established.
RSA key fingerprint is 97:8c:1b:f2:6f:14:6b:5c:3b:ec:aa:46:46:74:7c:40.
Are you sure you want to continue connecting (yes/no)?
Type yes and press Enter. The host key for bitbucket.org will now be added to the ~/.ssh/known_hosts file and you won't get this error in Jenkins anymore.
This helped me, I hope it helps you as well