Unable to create subtasks on iPhone/iPad by touching and holding a task

Good day: I create subtasks and I cannot mark them as complete when I’ve done the subtasks. They remain until/unless I delete them. I also cannot create subtasks on iPhone/iPad by touching and holding a task I want as a subtask and moving it up to the parent task. Subtasks seem more problem than they’re worth. I’m running the latest software and I’ve restarted all devices. Thank you.

[Re-Titled by Moderator]