We currently don't support the ability to create tasks in Outlook for iOS. For now, you can start by creating tasks in Outlook for Windows or Web.
You can also install the Microsoft To Do app to get started.
Feel free to give us feedback by going to Settings > Help & Feedback > Suggest a feature > Post an idea.
You can create tasks in many ways.
Creating tasks from Outlook for Web, or Windows or with Microsoft To Do: