Error Message: Duplicate Name Exists error of type: Vendor. The name supplied already exists.

This generally means you have a vendor of the same name as a customer.

Quickbooks Online doesn't allow for a Company Account to be a Customer (Client) and a Vendor. 

Solution

You will need to ensure that the company records are separated.

If you have vendors that are also customers, you will need to create a new separate company in ConnectWise, for example: XYZ Company (Vendor).