Have you debugged and stepped through this? The error says the file doesn't exist...so that means at some point you are setting an nRec file that isn't really there.
I'm wondering about your loop. You have
For i As Integer = 0 To _dtCol.Rows.Count
but I'm thinking you really want
For i As Integer = 0 To _dtCol.Rows.Count - 1
If you have 5 rows in your table, your loop would run 0, 1, 2, 3, 4, 5. Is there really a row with an index 5?....but this still doesn't make sense to me because then I think you'd get the error at this line:
nAcctNo = _dtCol.Rows(i).Item(0)
because it wouldn't find row 5.
You really need to debug and step through to see what is happening. Set a breakpoint and walk through the loop using F10. Check and make sure those variables for nRec and tRec are getting set the way you think they should.