[Answered ]-How to resolve this "Invalid default value for a django.models field error"?

You cannot have DAYS= 'D' as default, because it’s not tuple. Change the field to that:

deadline_type= models.CharField(max_length=16, choices=STATUS, default=STATUS[0], verbose_name="Days/Hours")

In that way you can delete DAYS= 'D' HOURS= 'H' and default value will be always the first tuple from STATUS.