You must configure OCC Settings to use the Online Contract Collaboration features and disable clickjack protection (if enabled) to view Visualforce pages.
Prerequisite
You must unselect the ClickjackProtection options to prevent errors while using OCC.
To disable Clickjack Protection
Go to Setup > Security > Session Settings.
Unselect:
Enable clickjack protection for customer Visualforce pages with standard headers
Enable clickjack protection for customer Visualforce pages with headers disabled
Click Save.
To configure OCC settings
Go to Setup > Custom Code > Custom Settings > OCC Settings > Manage > New.