Why I don't see any related tasks on activity in the P6 iOS application?

Click on the activity in the list of activities. Also, this application is referred to as the Team Member app. So you need to be an activity owner, primary resource or project manager with rights to the activity. It is not the all-inclusive P6 EPPM application that you use on the desktop. This application is for team members to update their tasks that they are responsible for or assigned to or are the owner of. Related tasks can be accessed after selecting the respective activity.