I cannot move to-dos to the Someday or Scheduled lists (FAQ)
A to-do that is not part of a project can be moved anywhere.
To-dos that are already part of a project have a few limitations. If they are part of an active project, they are not allowed to be in Someday or Scheduled. You can only set the whole project to inactive (move the whole project to Someday or Scheduled). See How can I set a project to inactive? (FAQ)
This limitation will be revised in future updates.
