I tried to remove one datatable in a way to keep only the Vendor one however still getting the same problem.
In fact, I have set the datasource of the ReportDocument to the one containing the datatable in question and filled in the datatable with results at runtime.

The datatable name is VendorList and it contains some columns. The next step I did is to create a new detail section and dragg each column once into the section.
Am I missing anything? How does the designer knows that this is a datatable format?