Is this a one-shot scenario or are there multiple instances where you'd need to check the due date of one item against another?
If it's just this one document name, then you can do a straight-up comparison like you're trying. You'd need a lot more code though because you have to access information in the database, catch errors, etc.
Assuming it's a straight-up comparison where you ONLY need to check the due date of "Panel Outline & Layout" against the due date of "Panel Preliminary Outline" then it's pretty simple.
Code:
' Assume rstTable is a Recordset of your Table (with the proper Filter),
' DueDate is the name of the Date field in your Form,
' DocName is the name of the Document Name field in your Form,
' tblDueDate is the name of the Due Date field in your Table, and
' tblDocName is the name of the Document Name field in your Table
If Me!DocName = "Panel Outline & Layout" Then
If rstTable("tblDueDate") = "" Or IsNull(rstTable("tblDueDate")) Then
With rstTable
.Edit
!tblDueDate = Me!DueDate
.Update
End With
End If
End If