Please change your code like below
Private Sub autoincrement()
getconnect()
If con.State = ConnectionState.Closed Then
con.Open()
End If
ds.Tables.Add(dt)
If ds.Tables.Rows.Count <> 0 Then
Dim reg_id As Double = 0
reg_id = ds.Tables(0).Compute("Max(registrationID)", Nothing)
TextBox14.Text = reg_id + 1
Else
TextBox14.Text = "100000"
End If
End Sub