Exam 156-315.80 topic 1 question 327 discussion

An administrator is creating an IPsec site-to-site VPN between his corporate office and branch office. Both offices are protected by Check Point Security Gateway managed by the same Security Management Server. While configuring the VPN community to specify the pre-shared secret the administrator found that the check box to enable pre-shared secret and cannot be enabled.
Why does it not allow him to specify the pre-shared secret?