When you can't work on the task that is already assigned to you, you need to unassign this task from self. This way you make this task accessible for other team members. They can work on this task instead of you.
Open Task details page
Click on "Unassign" button below task name
Please note: If you need to reassign task to a specific user, ask Process manager to do it. Only process manager has permissions to reassign task to another user.