Cannot Assign User to To Do List item
When you first install the module, I wouldn't be surprised if you'd want to create a To Do List item right away and then assign the task to one or more users.
It won't work.
Before users can be assigned a To Do List item, you must give them permission to be assigned to such an item. This is done in the usual permission place: Administer » User management » Permissions. You may also want to create a new role of people who deal with To Do List items.
Actually, the To Do List module comes with many permissions so you may want to ...