Why can't I assign a To do to a particular calendar?

Calendars are type-specific. Each calendar may contain either Events or To Dos, but not both.

When creating a Task or moving an existing one to a calendar, you must first ensure you have a Task Calendar available or the calendar you're moving to supports Tasks.

You can learn more about the types of Calendars here

assign todos to calendars. To do calendar. Cannot add to do. Cannot create task. To Do not adding. To do creating in wrong calendar. Cannot create new task todo to do.