终端复制 larabbs 源码的时候出错，有人知道怎么回事吗？

vagrant ssh
Welcome to Ubuntu 16.04.2 LTS (GNU/Linux 4.4.0-81-generic x86_64)

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151 packages can be updated.
42 updates are security updates.

Last login: Mon Apr 9 13:39:43 2018 from 10.0.2.2
vagrant@homestead:~$ git clone git@github.com:summerblue/larabbs.git
Cloning into 'larabbs'...
The authenticity of host 'github.com (52.74.223.119)' can't be established.
RSA key fingerprint is SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.com,52.74.223.119' (RSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.