If the Find function cannot match the string requested it returns Nothing and then you cannot set the due date from a variable that is Nothing. If this is a condition expected from the input then you need to protect the assignment to the new column with something like this.
For Each rw In ds.Tables(0).Rows
Dim strsearch As String = rw(0).ToString
Dim fequnumb As item = myItemList.FirstOrDefault(Function(p) p.EquipNumber = strsearch)
if fequnumb IsNot Nothing Then
rw(2) = fequnumb.DueDate <- Error occurs here
End if
Next
If this is not supposed to happen then you need to check your inputs