Thank you for asking. The availability of activities in the Planning phase depends on your project management scheme, which was selected in the activity Revise Project Complexity Assessment under the Initiation phase:
There are four schemes: “Basic”, “Low,” “Medium,” and “High”. Each scheme correspond to a specific level of project complexity. The activities and deliverables required to complete throughout the project are controlled by selection of scheme. While it’s recommended that project manager should choose a scheme based on the result of project complexity assessment, it’s up to his/her choice to select the scheme he/she want if deemed beneficial to the success of the project.
If you want to have all activities enabled, the scheme selection has to be High.