ERROR: Failed to add Invoice. Could not process Customer.

If you receive this error, go to the Client associated with the invoice to ensure that they are connected to Quickbooks properly.  

On the Client page in Centerbase check to see if the Quickbooks icon in the top right is red. This means that the client is not available in Quickbooks. Click the 'Send' button next to the colored icons in the top right to make the client available in Quickbooks.