Reverse your For Each Row and For Each. Put the For Each in the For Each Row in Datatable.
Inside the For Each, put an If with String.IsNullOrEmpty(CurrentRow(currentItem.ColumnName).ToString) as your condition. Inside the If, put an Assign with CurrentRow(currentItem.ColumnName) = currentItem.ColumnName
Make sure the TypeArgument of your For Each is System.Data.DataColumn.
Firstly, Get the row and column indices of the Missing Cell values. We can use the below Expression :
missingRowColumnTupleInfo = (From x In DT.AsEnumerable
From y In x.ItemArray
Where String.IsNullOrWhiteSpace(y.ToString)
Select Tuple.Create(DT.Rows.IndexOf(x),x.ItemArray.ToList.IndexOf(y))).ToArray
Here, missingRowColumnTupleInfo is a variable of Type Array of Tuple(Of Integer,Integer)
Next, we can loop the create Tuple Array which will consists of row and Column Indices of the Missing cell values and update the Datatable on those indices accordingly :