Quickbooks Error Code (6250): "The customer you have specified has been deleted."
For a full list of Quickbook Error Codes click here.
A Contact in QBO with an identical Display Name has been Archived.
Un-archive the Customer in Quickbooks, to do that follow these steps:
Select Sales or Invoicing from the left menu.
Select Customers at the top.
At the top of the customer list click the small gear and select Include Inactive.
Click the customer you wish to restore; inactive customer will be noted as (deleted).
Click the Make active button next to the customer name.