I have had to look at your previous question to understand your question.
The variable dt2 is a DataTable with only one column called "Position", so adding a DataGridViewRow to the DataRow collection of this DataTable makes no sense.
Your first loop should be
For n = 0 To dt.Rows.Count - 1
Dim r = dt2.NewRow();
r("Position") = dt.Rows.Item(n)("Mouse Position")
dt2.Rows.Add(r)
Next
while the second loop
For Each selectedItem As DataGridViewRow In selectedItems
Dim r = dt2.NewRow()
r("Position") = dt.Rows(selectedItem.Cells(1).Value.ToString)("Mouse Position")
dt2.Rows.Add(r)
Next