Update Record error

Update Record error

I have a task form that contains DueDate and also a Priority field.  What I'd like to do is to arrange it so that when the DueDate is approaching within X weeks or days, it will automatically update the Priority field to "1-High"  Ultimately (which I'm not sure if it can be done) my employer would like the form to auto adjust the priority not only based on the DueDate but also on the EstimatedHours for the project.  For example, if it will take an estimated 100 hours to complete a project, the project will automatically adjust the Priority to "1-High" 200 hours before the DueDate (doubling the EstimatedHours).

I thought perhaps scheduling it would work best so I've been trying to define the action but although it is allowing me to fetch all records that fit within the criteria, it isn't allowing me to update the record Priority, claiming the field Priority doesn't exist.  However, both fields are in the same form.

  1. duedate = Add_Task[DueDate < (zoho.currentdate  +  '3W:2D:0H:0M:0S')].DueDate;
  2. duedate.Priority="1-High";

Any idea what I am doing wrong?