I suppose you mean the "Create Test Plan" option is not available in the "Planning" menu.  The logged in user must be in a user role that has the permission to create test plan for the project area.  For example, the out-of-box Test Team Member user role.  Or the project admin explicitly configured the Menu Organization settings to have the menu option hidden.  You should be able to get more info by comparing the project area settings of one that has the menu option to one that does not have the menu option.